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

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

Add NetBSD's mtree to the tree and install it as nmtree as the first step
towards replacing our mtree.

Sponsored by: DARPA, AFRL
Thanks to: cristos@NetBSD for reviewing and committing my patches
wiz@NetBSD for fixing typos in my patches

Vendor import of NetBSD's mtree at 2012-12-21