Mergemaster tweak

I never have /usr/src on my system (other than the directory that mtree
creates). So when I run mergemaster it's always failing until I remember to
add the -m . (or was it -t ., I sometimes forget).

Given the recent ntpd stuff, I was annoyed enough with this default
behavior to create a  fix for it, which I've uploaded to
https://reviews.freebsd.org/D16709 for review.

Basically, if the proposed SOURCEDIR doesn't have a Makefile.inc1, but .
does, then it prompts to use that. If it can't find one, it errors out
early as a nice side effect.

Please comment on the review. Thanks!

Warner