Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a short regression test to try to exercise races in the non-atomic
nature of implied connect via sendto(). Oddly, uipc_usrreq.c implements
this for stream sockets, but doesn't set the flag in its protocol
definition so that it can actually be used. As such, the stream test is
implemented but doesn't run for now.