MFC r257138:

Switch the default mtree to nmtree our new NetBSD derived mtree.

Exp-runs by:	bdrewery (ports/182438)
Sponsored by:	DARPA/AFRL
Approved by:	re (gjb)
Notional MFC of r257139:

Regerate after r257460 swapped the default to WITH_NMTREE.

Approved by:	re (gjb)