Use program_invocation_short_name in --help
Add compatibility wrappers for the MSYS runtime

It should be possible to build ssh-pageant on MSYS, but since that's
basically an old snapshot of Cygwin, it's missing some modern
functionality.

Introduce compat.h with the following wrappers:
- program_invocation_short_name
- err(), errx(), warn(), warnx()
- mkdtemp()
- strlcpy()
- SOCK_CLOEXEC in accept4() and socket()

This should have zero net effect on Cygwin builds.
Polish compat.h edge cases

- mkdtemp: Use the returned path for mkdir, likely the same anyway.
- strlcpy: Check the size before writing the zero terminator.
- accept4: Set an error if unsupported flags are used.

None of these problems are present in the way ssh-pageant uses them, but
it's better to be safe.