Revise locking strategy used for UNIX domain sockets in order to improve

concurrency:

- Add per-unpcb mutexes protecting unpcb connection state, fields, etc.

- Replace global UNP mutex with a global UNP rwlock, which will protect the
  UNIX domain socket connection topology, v_socket, and be acquired
  exclusively before acquiring more than per-unpcb at a time in order to
  avoid lock order issues.

In performance measurements involving MySQL, this change has little or no
overhead on UP (+/- 1%), but leads to a significant (5%-30%) improvement in
multi-processor measurements using the sysbench and supersmack benchmarks.

Much testing by:	kris
Approved by:		re (kensmith)

sys/sys/unpcb.h

@@ -79,6 +79,7 @@ struct unpcb {
 	int	unp_flags;		/* flags */
 	struct	xucred unp_peercred;	/* peer credentials, if applicable */
 	u_int	unp_refcount;
+	struct	mtx unp_mtx;		/* mutex */
 };
 
 /*