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

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

Add the netgraph 'source' module.
This is NOT YET CONVERTED TO -current.
This node is a source for preprogrammed packets at a known rate for testing.

I will convert it to -current "in place" but will MFC teh original
pre-conversion variant as that is what is originally submitted.
Man page my me, info from Dave's README.

Submitted by: Dave Chapeskie <dchapeskie@SANDVINE.com>
Obtained from: Sandvine inc.
MFC after: 1 week