Server behaviour tests, running on every platform (incl. Windows)

[hhugo]

Adds a suite of behaviour tests for the server and makes the whole suite — including the serve-mode HTTP tests — pass on every platform, including Windows in bytecode. Builds on the Windows-CI work in #253.

Tests

• http-semantics — methods (GET/HEAD), status codes, content-type/length
• static-files — index resolution, nested paths, directory listing, protection against escaping the served directory
• range-requests — documents that byte ranges are currently ignored (TODO where Ocsigen_range used to be in ocsigen_cohttp.ml)
• serve / serve-listing / serve-cli — end-to-end serve-mode binary: directory listing on/off + CLI error handling
• extensions — redirectmod, accesscontrol, authbasic, cors, rewritemod, outputfilter (one focused test each)

Harness / CI

• dune runtest is now wired into CI (it was never run before)
• server-test-helpers.sh rewritten in POSIX sh (was bash-only, failed under dash); server logs go to a file to avoid non-deterministic interleaving
• serve-mode servers are stopped gracefully (ocsigenserver --command shutdown) so they exit on their own — nothing is left for dune's cram cleanup to hang on, which lets the serve-mode tests run on Windows too
• programmatic (unix-socket) tests are skipped on Windows via (enabled_if (<> %{os_type} Win32))
• fixed bitrotted tests: deflatemod test.ml used pre-modernisation module names; serve.t had a spurious (no-eol)

Windows portability fixes (needed to run the suite there)

• secure RNG (Cryptokit.Random.secure_rng) instead of opening /dev/urandom
• tolerate signals (e.g. SIGPIPE) that don't exist on Windows
• Content-Length from stat instead of Lwt_io.length, which seeks and fails on Windows
• local_files: fix a Filename.dirname infinite loop at drive roots (e.g. D:\) that wedged the Lwt main thread on directory requests — this was the real cause of the directory-listing hang
• trim the trailing CR from ocamlfind output when computing builtin packages

Command channel

• a new --command CMD --command-pipe NAME client mode lets a running server be controlled (notably stopped) without a signal
• the command pipe is a FIFO on Unix and a polled regular file on Windows (where mkfifo is unavailable), so graceful shutdown works everywhere with no platform-specific C code

A named-pipe variant of the command channel (plus a couple of changes that turned out unnecessary) is preserved on the windows-serve-support branch.

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