Push solock() down to sosend() and remove it from soreceive() paths fro

Push solock() down to sosend() and remove it from soreceive() paths fro
unix(4) sockets.

Push solock() deep down to sosend() and remove it from soreceive() paths
for unix(4) sockets.

The transmission of unix(4) sockets already half-unlocked because
connected peer is not locked by solock() during sbappend*() call. Use
`sb_mtx' mutex(9) and `sb_lock' rwlock(9) to protect both `so_snd' and
`so_rcv'.

Since the `so_snd' is protected by `sb_mtx' mutex(9) the re-locking
is not required in uipc_rcvd().

Do direct `so_rcv' dispose and cleanup in sofree(). This sockets is
almost dead and unlinked from everywhere include spliced peer, so
concurrent sotask() thread will just exit. This required to keep locks
order between `i_lock' and `sb_lock'. Also this removes re-locking from
sofree() for all sockets.

SB_OWNLOCK became redundant with SB_MTXLOCK, so remove it. SB_MTXLOCK
was kept because checks against SB_MTXLOCK within sb*() routines are mor
consistent.

Feedback and ok bluhm