mtree acl support

[Mark Felder]

I was recently talking to someone about how one would backup / restore
ACLs reliably. I didn't see any mention of ACLs in the mtree man page
and after a quick google I came upon this old mailing list post:

http://lists.freebsd.org/pipermail/freebsd-hackers/2008-April/024173.html

patch in list is here: http://heka.cenkes.org/sat/diffs/mtree_acl.diff
I've mirrored it here: https://feld.me/freebsd/mtree_acl.diff

This old patch appears to still apply cleanly. I hate to see a patch die
and be forgotten.