MFC r258437:

Sync with NetBSD.  The functional change is to make the output when
comparing a directory to an mtree file more compatible with fmtree when
FreeBSD 9 compatiblity mode is on.  This output is clearly intended for
humans not computers, but some tools such as mergemaster's -U option rely
on it.

Approved by:	re (glebius)
MFC: r258456

Fix mergemaster -U by forcing FreeBSD 9 compatiblity in mtree when mtree is
nmtree.

The mtree output used by mergemaster in this case was clearly not meant for
computer consumption and an approach based on -f <file1> -f <file2> would
probably be a better idea, but this is a minimal change.

Approved by:	re (glebius)