socket: preserve real errno when socket/bind calls fail

socket: preserve real errno when socket/bind calls fail

When reporting socket/bind failures we want to ensure any fatal error
reported is as accurate as possible. We'll prefer reporting a bind()
errno over a socket() errno, because if socket() works but bind() fails
that is a more significant event.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>