package vs. MTREE, bsd.port.mk patch, request for comments

[Marc Espie <espie_(_at_)_schutzenberger_(_dot_)_liafa_(_dot_)_jussieu_(_dot_)_fr>]

This adds checks to bsd.port.mk to make sure that we use the right version
of /etc/mtree.

Yes, this means that each time the ports tree tracks a new snapshot,
someone (turans@, most probably) will have to update MTREE_VERSION, 
but this is better than allowing people to build package with the wrong
mtree files.

Of course, getting smarter about mtree handling would also solve this...