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

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

Add some more cross-references.

Add a man page describing the feed-forward clock kernel support, including how
to enable and configure the functionality.

Committed on behalf of Julien Ridoux and Darryl Veitch from the University of
Melbourne, Australia, as part of the FreeBSD Foundation funded "Feed-Forward
Clock Synchronization Algorithms" project.

For more information, see http://www.synclab.org/radclock/

Discussed with: Julien Ridoux (jridoux at unimelb edu au)
Submitted by: Julien Ridoux (jridoux at unimelb edu au)