Update dirdeps.mk et al

dirdeps.mk: move logic to handle -f dirdeps.mk to inside check
for first read of dirdeps.mk

Also fix handling of WITHOUT_DIRDEPS_BELOW

gendirdeps.mk: pass M2D_EXCLUDES to meta2deps

meta.autodep.mk: if we build with nofilemon, leave a cookie to
prevent updating dependencies until cleaned.

Reviewed by: bdrewery