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

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

Merge mandoc from vendor into contrib and provide the necessary Makefile glue.

It's not yet connected to the build.

Vendor import of mdocml 1.12.1

It is a suite of ISC licensed tools to compile and render mdoc/man pages and
will replace groff for formatting manpages in the base system.

http://mdocml.bsd.lv/