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

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

Remove trailing whitespace per mdoc lint warning

Disussed with: gavin
No objection from: doc
Approved by: joel
MFC after: 3 days

mdoc: Remove tabs in non-literal context.

Remove superfluous paragraph macro.

[mdoc] Fixed .Dt call.

Add a manpage for the nvram2env driver.