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

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

mdoc: Avoid playing tricks with Ns: If Nm is present in the SYNOPSIS section,
it will be output on its own line. Ns cancels this effect however. This change
is also consistent with the rest of our manual pages.

mdoc: drop redundant .Pp and .LP calls

They have no effect when coming in pairs, or before .Bl/.Bd

Manual page for the kernel side Secure Neighbor Discovery support.

Reviewed by: brueffer
Approved by: bz (mentor)