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

Use '%zu' and '%zd' as appropriate for size_t / ssize_t.

Update udpzerobyte to understand that passing 0 as a length to recv(2)
will cause it to return 0, not EAGAIN.

Add UNIX domain socket support to udpzerobyte, which suggests this
regression test should be moved to the general sockets test area rather
than netinet.

Add IPv6 support to zero-size UDP transmit/receive test.

Regression test for the loopback handling of zero-length UDP packets, which
should be delivered but without payload.