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

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

Minor man page cleanup -- fix grammar, markup fixes, and start each
sentence on a new line.

Approved by: hrs (mentor)

mdoc: remove superfluous paragraph macro.

Update netmap page, fixing the API documentation and usage example.

Add a new manpage for the vale switch