Vendor import of NetBSD's mtree at 2013-04-08.
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
Use IP6STAT_INC/IP6STAT_DEC macros to update ip6 stats.

MFC after:	1 week