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

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

Import a new version of NetBSD's mtree.

This version of mtree implements a new flag (-O) that can be used to
restrict the tool to certain pathnames. Also, it fixes a compiler
warning generated by -Wmissing-variable-declarations.

Acked by: brooks

Vendor import of NetBSD's mtree at 2013-04-08.