MFC r263116

Replace 4.4BSD Lite's unix domain socket backpressure hack with a cleaner
mechanism, based on the new SB_STOP sockbuf flag.  The old hack dynamically
changed the sending sockbuf's high water mark whenever adding or removing
data from the receiving sockbuf.  It worked for stream sockets, but it never
worked for SOCK_SEQPACKET sockets because of their atomic nature.  If the
sockbuf was partially full, it might return EMSGSIZE instead of blocking.

The new solution is based on DragonFlyBSD's fix from commit
3a6117bbe0ed6a87605c1e43e12a1438d8844380 on 2008-05-27.  It adds an SB_STOP
flag to sockbufs.  Whenever uipc_send surpasses the socket's size limit, it
sets SB_STOP on the sending sockbuf.  sbspace() will then return 0 for that
sockbuf, causing sosend_generic and friends to block.  uipc_rcvd will
likewise clear SB_STOP.  There are two fringe benefits: uipc_{send,rcvd} no
longer need to call chgsbsize() on every send and receive because they don't
change the sockbuf's high water mark.  Also, uipc_sense no longer needs to
acquire the UIPC linkage lock, because it's simpler to compute the
st_blksizes.

There is one drawback: since sbspace() will only ever return 0 or the
maximum, sosend_generic will allow the sockbuf to exceed its nominal maximum
size by at most one packet of size less than the max.  I don't think that's
a serious problem.  In fact, I'm not even positive that FreeBSD guarantees a
socket will always stay within its nominal size limit.

sys/sys/sockbuf.h
	Add the SB_STOP flag and adjust sbspace()

sys/sys/unpcb.h
	Delete the obsolete unp_cc and unp_mbcnt fields from struct unpcb.

sys/kern/uipc_usrreq.c
	Adjust uipc_rcvd, uipc_send, and uipc_sense to use the SB_STOP
	backpressure mechanism.  Removing obsolete unpcb fields from
	db_show_unpcb.

tests/sys/kern/unix_seqpacket_test.c
	Clear expected failures from ATF.