Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r244562,245241,245435

Add NetBSD's mtree to the tree and install it as nmtree.

Always install our mtree as /usr/sbin/fmtree and link it as
/usr/sbin/mtree by default.

Add a src.conf option WITH_NMTREE that causes NetBSD's mtree to be linked
as /usr/sbin/mtree as well as /usr/sbin/nmtree.

Sponsored by: DARPA, AFRL

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