Hook the meta/nofilemon build into using FAST_DEPEND.

FAST_DEPEND is intended to be the "skip 'make depend' and mkdep"
feature. Since DIRDEPS_BUILD does this already with some of its own
hacks, and filemon doesn't need this, and nofilemon does, teach it how
to handle each of these cases.

In meta+filemon mode filemon will handle dependencies itself via the
meta mode logic in bmake. We still want to set MK_FAST_DEPEND=yes to
enable some logic that indicates that 'make depend' is skipped in the
traditional sense. The actual .depend.* files will be skipped.

When nofilemon is set though we still need to track and generate dependencies.

Sponsored by: EMC / Isilon Storage Division