Windows: add a wrapper for the shutdown() system call

Windows: add a wrapper for the shutdown() system call

Even though Windows's socket functions look like their POSIX counter parts,
they do not operate on file descriptors, but on "socket objects". To bring
the functions in line with POSIX, we have proxy functions that wrap and
unwrap the socket objects in file descriptors using open_osfhandle and
get_osfhandle. But shutdown() was not proxied, yet. Fix this.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>