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

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

mdoc: order prologue macros consistently by Dd/Dt/Os

Although groff_mdoc(7) gives another impression, this is the ordering
most widely used and also required by mdocml/mandoc.

Reviewed by: ru
Approved by: philip, ed (mentors)

Sort sections.

Markup fixes.

Approved by: re (blanket)

SEE ALSO is the special name, and should not be taken in quotes,
since -mdoc checks for the "SEE" word only to set its variables.
In the SEE ALSO section, -mdoc automatically emits an empty line
before each new reference.

Prepare for mdoc(7)NG.

Fix up this for the build; some compatibility files have been added -
these will be submitted back to KTH for inclusion.