Fix PR kern/185813 "SOCK_SEQPACKET AF_UNIX sockets with asymmetrical
buffers drop packets".  It was caused by a check for the space available
in a sockbuf, but it was checking the wrong sockbuf.

sys/sys/sockbuf.h
sys/kern/uipc_sockbuf.c
    Add sbappendaddr_nospacecheck_locked(), which is just like
    sbappendaddr_locked but doesn't validate the receiving socket's
    space.  Factor out common code into sbappendaddr_locked_internal().
    We shouldn't simply make sbappendaddr_locked check the space and
    then call sbappendaddr_nospacecheck_locked, because that would cause
    the O(n) function m_length to be called twice.

sys/kern/uipc_usrreq.c
    Use sbappendaddr_nospacecheck_locked for SOCK_SEQPACKET sockets,
    because the receiving sockbuf's size limit is irrelevant.

tests/sys/kern/unix_seqpacket_test.c
    Now that 185813 is fixed, pipe_128k_8k fails intermittently due to
    185812.  Make it fail every time by adding a usleep after starting
    the writer thread and before starting the reader thread in
    test_pipe.  That gives the writer time to fill up its send buffer.
    Also, clear the expected failure message due to 185813.  It actually
    said "185812", but that was a typo.

PR:		kern/185813
Reviewed by:	silence from freebsd-net@ and rwatson@
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corporation
Only use -lpthread for unix_seqpacket_test, not for all test programs.

Reported by:	Garrett Cooper
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corporation