r4831: added udp support to our generic sockets library.

r4831: added udp support to our generic sockets library.

I decided to incorporate the udp support into the socket_ipv4.c
backend (and later in socket_ipv6.c) rather than doing a separate
backend, as so much of the code is shareable. Basically this adds a
socket_sendto() and a socket_recvfrom() call and not much all.

For udp servers, I decided to keep the call as socket_listen(), even
though dgram servers don't actually call listen(). This keeps the API
consistent.

I also added a simple local sockets testsuite in smbtorture,
LOCAL-SOCKET
(This used to be commit 9f12a45a05c5c447fb4ec18c8dd28f70e90e32a5)