Unix: Add _PATH_BSHELL and _PATH_DEFPATH where available. The following are sources to each platform's paths.h:

From BSD:
* [Apple](https://github.com/apple-oss-distributions/Libc/blob/71bbe350ab79eef58113991d817ccc6165061a64/include/paths.h#L65): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [Dragonfly](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/42aaacafd14ddb7f660730a5ca5da0a2d941723a/include/paths.h#L40): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [FreeBSD](https://github.com/freebsd/freebsd-src/blob/551b7c5e12bfea623a47edf97ad5689732a1233f/include/paths.h#L40): Has `_PATH_BSHELL` and `_PATH_DEFPATH` (verified same values on version 11+)
* [NetBSD](https://github.com/NetBSD/src/blob/f66621237dc60126bd8a972b6064639892b344b7/include/paths.h#L43): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [OpenBSD](https://github.com/openbsd/src/blob/7d89a3e823169cb6a1d989e3f5f93f7f3c4b5db4/include/paths.h#L39): Has `_PATH_BSHELL` and `_PATH_DEFPATH`

From Linux:
* [Android](https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/include/paths.h#48): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [glibc](https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/paths.h;h=1342ab3a96ab12065311b718e57e7235e07587f8;hb=HEAD#l36): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [musl](https://git.musl-libc.org/cgit/musl/tree/include/paths.h?id=f21a96538f78fa8e2040831b4209b35f2fb581da#n4): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [uClibc](https://github.com/kraj/uClibc/blob/ca1c74d67dd115d059a875150e10b8560a9c35a8/include/paths.h#L36): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [emscripten](https://github.com/emscripten-core/emscripten/blob/a2059f978ed206b688daf4a8774211949610c2aa/system/lib/libc/musl/include/paths.h#L4): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* l4re: There's [musl](https://github.com/l4re/l4re-core/blob/d09b1619c45d17786bc256ea219688f8ba3ff1fe/libc/musl/contrib/musl/include/paths.h#L4) and [uclibc](https://github.com/l4re/l4re-core/blob/d09b1619c45d17786bc256ea219688f8ba3ff1fe/libc/uclibc-ng/contrib/uclibc/include/paths.h#L36) `paths.h` file containing `_PATH_BSHELL` and `_PATH_DEFPATH` values

Miscellaneous Platforms:
* [Haiku](https://github.com/haiku/haiku/blob/8a33129223e93ea046dc62e9e09f8c93f45cfb7b/headers/compatibility/bsd/paths.h#L45): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [Hurd](https://github.com/joshumax/hurd/blob/83a6fc7641eecdb2b96c2dee2261da92573693de/hurd/paths.h#L20): Doesn't have `_PATH_BSHELL` and `_PATH_DEFPATH` in paths.h (wouldn't hurt to confirm this though in terminal)
* [Cygwin](https://github.com/cygwin/cygwin/blob/cf61e0140e3d6cd21e743a38d35efc98194b63f3/winsup/cygwin/include/paths.h#L12): Has `_PATH_BSHELL` and `_PATH_DEFPATH`
* [newlib](https://github.com/ourairquality/newlib/blob/0dea38754696422d526543262274703acd8317c5/newlib/libc/include/paths.h#L7): Has `_PATH_BSHELL`
* [Redox](https://gitlab.redox-os.org/redox-os/relibc/-/blob/c3b3f2773a3a2622fdb632a9bfa1a77d68778150/include/paths.h): Has `_PATH_BSHELL`

Platforms I'm unsure if they have `_PATH_BSHELL`/`_PATH_DEFPATH`:
* Solarish: I'm unsure how to get this info since I believe Solaris is closed source?
* AIX: I think this is also closed source? However, I saw this [archive repo](https://github.com/Arquivotheca/AIX-4.1.3/blob/d6fe8fe8299ececc0db4fae9c19cd3babffd28ae/bos/usr/include/paths.h#L36) of AIX 4.13 that has `_PATH_BSHELL`.
* [nuttx](https://github.com/apache/nuttx): Couldn't find a `include/paths.h` file.
* [NTO/QNX](https://github.com/qnx): Couldn't find a `include/paths.h` file.

Author: asder8215

libc-test/build/main.rs

@@ -200,6 +200,7 @@ fn test_apple(t: &Target) {
         "os/lock.h",
         "os/signpost.h",
         "os/os_sync_wait_on_address.h",
+        "paths.h",
         "poll.h",
         "pthread.h",
         "pthread_spis.h",
@@ -473,6 +474,7 @@ fn test_openbsd(t: &Target) {
         "sys/shm.h",
         "sys/param.h",
         "sys/auxv.h",
+        "paths.h",
     );
 
     cfg.rename_type(|ty| match ty {
@@ -574,6 +576,7 @@ fn test_cygwin(t: &Target) {
         "net/if.h",
         "netdb.h",
         "netinet/tcp.h",
+        "paths.h",
         "poll.h",
         "pthread.h",
         "pty.h",
@@ -875,6 +878,7 @@ fn test_redox(t: &Target) {
         "netinet/in.h",
         "netinet/ip.h",
         "netinet/tcp.h",
+        "paths.h",
         "poll.h",
         "pwd.h",
         "semaphore.h",
@@ -1303,6 +1307,7 @@ fn test_netbsd(t: &Target) {
         "iconv.h",
         "utmp.h",
         "utmpx.h",
+        "paths.h",
     );
 
     cfg.rename_type(move |ty| {
@@ -1546,6 +1551,7 @@ fn test_dragonflybsd(t: &Target) {
         "netinet/ip.h",
         "netinet/tcp.h",
         "netinet/udp.h",
+        "paths.h",
         "poll.h",
         "pthread.h",
         "pthread_np.h",
@@ -2077,6 +2083,7 @@ fn test_android(t: &Target) {
         // generate the error 'Your time_t is already 64-bit'
         (t.p32(), "time64.h"),
         (x86, "sys/reg.h"),
+        "paths.h",
     );
 
     // Include linux headers at the end:
@@ -2560,6 +2567,7 @@ fn test_freebsd(t: &Target) {
         "netinet/tcp.h",
         "netinet/udp.h",
         "netinet6/in6_var.h",
+        "paths.h",
         "poll.h",
         "pthread.h",
         "pthread_np.h",
@@ -3247,6 +3255,7 @@ fn test_emscripten(t: &Target) {
         "utmp.h",
         "utmpx.h",
         "wchar.h",
+        "paths.h",
     );
 
     cfg.rename_struct_ty(move |ty| {
@@ -4106,6 +4115,7 @@ fn test_linux(t: &Target) {
         // https://www.openwall.com/lists/musl/2015/04/09/3
         // <execinfo.h> is not present on uclibc.
         (!(musl || uclibc), "execinfo.h"),
+        "paths.h",
     );
 
     // Include linux headers at the end:
@@ -5384,6 +5394,7 @@ fn test_haiku(t: &Target) {
         "netinet6/in6.h",
         "nl_types.h",
         "null.h",
+        "paths.h",
         "poll.h",
         "pthread.h",
         "pwd.h",

libc-test/semver/android.txt

@@ -3175,6 +3175,8 @@ X_OK
 _IOFBF
 _IOLBF
 _IONBF
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ALLOC_SIZE_MIN
 _PC_ASYNC_IO

libc-test/semver/apple.txt

@@ -1631,6 +1631,8 @@ _NSGetArgc
 _NSGetArgv
 _NSGetEnviron
 _NSGetProgname
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ALLOC_SIZE_MIN
 _PC_ASYNC_IO

libc-test/semver/cygwin.txt

@@ -472,6 +472,8 @@ WINDOWS_SEND
 XTABS
 _CS_PATH
 _IONBF
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ALLOC_SIZE_MIN
 _PC_ASYNC_IO

libc-test/semver/dragonfly.txt

@@ -1175,6 +1175,8 @@ _CS_PATH
 _IOFBF
 _IOLBF
 _IONBF
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ACL_EXTENDED
 _PC_ACL_PATH_MAX

libc-test/semver/espidf.txt

@@ -32,6 +32,7 @@ SIGSEGV
 SIGTERM
 SOL_SOCKET
 SOMAXCONN
+_PATH_BSHELL
 __errno
 cmsghdr
 dirent

libc-test/semver/freebsd.txt

@@ -1770,6 +1770,8 @@ _IOR
 _IOW
 _IOWINT
 _IOWR
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_ACL_EXTENDED
 _PC_ACL_NFS4
 _PC_ACL_PATH_MAX

libc-test/semver/linux.txt

@@ -3873,6 +3873,8 @@ _IONBF
 _IOR
 _IOW
 _IOWR
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ALLOC_SIZE_MIN
 _PC_ASYNC_IO

libc-test/semver/netbsd.txt

@@ -1132,6 +1132,8 @@ _IONBF
 _IOR
 _IOW
 _IOWR
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ACL_EXTENDED
 _PC_FILESIZEBITS

libc-test/semver/openbsd.txt

@@ -989,6 +989,8 @@ _IOR
 _IOW
 _IOWR
 _MAX_PAGE_SHIFT
+_PATH_BSHELL
+_PATH_DEFPATH
 _PC_2_SYMLINKS
 _PC_ALLOC_SIZE_MIN
 _PC_ASYNC_IO