Manually implement a socketpair() function, using UNIX sockets and
simple peer credential checking.

QEMU doesn't make much use of socketpair, beside vhost-user which is not
available for win32 at this point. However, I intend to use it for
writing some new portable tests.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230306122751.2355515-5-marcandre.lureau@redhat.com>