PR #5712 predates several refactorings to move the utility code into a
more general purpose codebase. Update to reflect the refactorings.

Fixes #5598

Monsters.

Not all systems have poll(2); emulate it with select(2).

Make socket I/O non-blocking and add optional timeouts.

Users may now set `GIT_OPT_SET_SERVER_CONNECT_TIMEOUT` to set a shorter
connection timeout. (The connect timeout cannot be longer than the
operating system default.) Users may also now configure the socket read
and write timeouts with `GIT_OPT_SET_SERVER_TIMEOUT`.

By default, connects still timeout based on the operating system
defaults (typically 75 seconds) and socket read and writes block.

Add a test against our custom testing git server that ensures that we
can timeout reads against a slow server.

The `gitno` buffer interface is another layer on top of socket reads.
Abstract it a bit into a "static string" that has `git_str` like
semantics but without heap allocation which moves the actual reading
logic into the socket / stream code, and allows for easier future usage
of a static / stack-allocated `git_str`-like interface.

We lose some error information from the read / write callbacks to
stransport. Store our own error value in the object so that we can
ensure that we rely upon it.