Modify UDP to use sosend_dgram() instead of sosend(). This allows

for signicantly optimized UDP socket I/O when using a single UDP
socket from many threads or processes that share it, by avoiding
significant locking and other overhead in the general sosend()
path that isn't necessary for simple datagram sockets.  Specifically,
this change results in a significant performance improvement for
threaded name service in BIND9 under load.

Suggested by:	Jinmei_Tatsuya at isc dot org

sys/netinet/udp_usrreq.c

@@ -1118,6 +1118,7 @@ struct pr_usrreqs udp_usrreqs = {
 	.pru_disconnect =	udp_disconnect,
 	.pru_peeraddr =		udp_peeraddr,
 	.pru_send =		udp_send,
+	.pru_sosend =		sosend_dgram,
 	.pru_shutdown =		udp_shutdown,
 	.pru_sockaddr =		udp_sockaddr,
 	.pru_sosetlabel =	in_pcbsosetlabel