net/socket: learn to talk with a unix dgram socket

-net socket has a fd argument, and may be passed pre-opened sockets.

TCP sockets use framing.
UDP sockets have datagram boundaries.

When given a unix dgram socket, it will be able to read from it, but
will attempt to send on the dgram_dst, which is unset. The other end
will not receive the data.

Let's teach -net socket to recognize a UNIX DGRAM socket, and use the
regular send() command (without dgram_dst).

This makes running slirp out-of-process possible that
way (python pseudo-code):

a, b = socket.socketpair(socket.AF_UNIX, socket.SOCK_DGRAM)

subprocess.Popen('qemu -net socket,fd=%d -net user' % a.fileno(), shell=True)
subprocess.Popen('qemu ... -net nic -net socket,fd=%d' % b.fileno(), shell=True)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>