support Handle to Handle sendfile

[joeyh]

At least on Linux, sendfile(2) operates on fds, so it should be possible to have eg:

sendFile :: Handle -> Handle -> IO ()

I need this and noticed it was missing in the current API of this library, which assumes that at least one end is a Socket.

Use cases include eg, sending a file to stdout as an efficient implementation of cat.

By the way, you're also missing a way to receive a file from a Socket and send it to a Handle.

[stepcut]

That is likely all true.

The initial aim of this library was to provide a consist API across Linux/Windows/Darwin/FreeBSD for the minimal sendfile functionality required to implement happstack-server.

I am not opposed to you adding additional functionality -- though adding functions that are only available on some OSes and not others raised questions about the PVP.