Make an attempt to detect missing MTREE files in distrib-dirs.  Not
perfect, but this is just a developer seatbelt.

PR:		conf/176897
Submitted by:	Garrett Cooper <yaneurabeya@gmail.com>
MFC after:	1 week
Fix DDP breakage introduced in r248925.  Bitwise OR has higher
precedence than ternary conditional.

MFC after:	1 week