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

Up the scale of the SPX loopback check a bit: use much larger data sizes
so that we need to do segmentation.

Add simple regression test to ping pong a data packet over SPX in stream
mode. Support both connection via connect() and sendto(), but don't
compile in sendto() for now, since netipx doesn't appear to actually
implement that (doh).