sbappendcontrol() was improperly setting sb_lastmbuf, creating a possible

sbappendcontrol() was improperly setting sb_lastmbuf, creating a possible
mbuf leak and/or sb_cc count mismatch.  The passed mbuf 'm' is a chain,
not a single mbuf, and must be iterated through to locate the last mbuf.

Optimize sbappend().  M_EOR is always set in the last mbuf and the sockbuf's
sb_lastmbuf and sb_lastrecord fields can be used instead of iterating
through all the mbufs in the sockbuf.  Only UNIX domain stream sockets used
this function.  Normal TCP connections use the already-optimized
sbappendstream() call.

Properly propogate M_EOR in sbappendcontrol() and sbappendaddr().

Do various code and comment cleanups.