The sync filter.

This adds a new filter for synchronization between clients. It works
by sending an UDP packet to other clients ("buddies").

To reduce latency, address resolution is only performed once on
startup.  Hence lookup_address() and makesock_addrinfo() of net.c
are made public.

This commit introduces new public function sockaddr_equal() in
net.c which compares two IPv4 or IPv6 addresses. It is used in
sync_find_buddy().