MFC r267667:
use .Mt to mark up email addresses consistently (part1)

PR: 191174
Submitted by: Franco Fichtner <franco at lastsummer.de>

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

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

Moved mcgrab(1) into a separate directory -- our .mk infrastructure
doesn't currently have support for building multiple programs in a
single makefile.

While here, fixed manpages and makefiles (missing dependencies).

Add a new program to the multicast test suite. The mcgrab program
is used to grab and hold some number of multicast addresses in order
to test what happens when an interface goes over the number of multicast
addresses it can filter in hardware.