udp: Restore UDP sockets

Reuse the TCP socket restore, just add connect() and sanity checks
for protocol.

This is OK, since UDP connect doesn't go to network for connection
and (unlike unix sockets) doesn't require peer to be "online". It
just puts the peer's creds on socket and returns.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>