First cut at implementing SOCK_SEQPACKET support for UNIX (local) domain

sockets.  This allows for reliable bi-directional datagram communication
over UNIX domain sockets, in contrast to SOCK_DGRAM (M:N, unreliable) or
SOCK_STERAM (bi-directional bytestream).  Largely, this reuses existing
UNIX domain socket code.  This allows applications requiring record-
oriented semantics to do so reliably via local IPC.

Some implementation notes (also present in XXX comments):

- Currently we lack an sbappend variant able to do datagrams and control
  data without doing addresses, so we mark SOCK_SEQPACKET as PR_ADDR.
  Adding a new variant will solve this problem.

- UNIX domain sockets on FreeBSD provide back-pressure/flow control
  notification for stream sockets by manipulating the send socket
  buffer's size during pru_send and pru_rcvd.  This trick works less well
  for SOCK_SEQPACKET as sosend_generic() uses sb_hiwat not just to
  manage blocking, but also to determine maximum datagram size.  Fixing
  this requires rethinking how back-pressure is done for SOCK_SEQPACKET;
  in the mean time, it's possible to get EMSGSIZE when buffers fill,
  instead of blocking.

Discussed with:	benl
Reviewed by:	bz, rpaulo
MFC after:	3 months
Sponsored by:	Google

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