Reduce the number of unnecessary unlock-relocks on socket buffer mutexes
associated with performing a wakeup on the socket buffer:

• When performing an sbappend*() followed by a so[rw]wakeup(), explicitly acquire the socket buffer lock and use the _locked() variants of both calls. Note that the _locked() sowakeup() versions unlock the mutex on return. This is done in uipc_send(), divert_packet(), mroute socket_send(), raw_append(), tcp_reass(), tcp_input(), and udp_append().

• When the socket buffer lock is dropped before a sowakeup(), remove the explicit unlock and use the _locked() sowakeup() variant. This is done in soisdisconnecting(), soisdisconnected() when setting the can't send/ receive flags and dropping data, and in uipc_rcvd() which adjusting back-pressure on the sockets.

For UNIX domain sockets running mpsafe with a contention-intensive SMP
mysql benchmark, this results in a 1.6% query rate improvement due to
reduce mutex costs.