Update to latest dirdeps.mk

Take advantage of new bmake feature to only consider Makefile.depend
as invalidating DIRDEPS_CACHE.

When bootstrapping allow more filtering via .MAKE.DEPENDFILE_BOOTSTRAP_SED

Move some comments back to where they make sense.

meta.sys.mk: add META_COOKIE_TOUCH and META_NOPHONY to better handle some
targets in meta mode vs non-meta mode.
Also use .MAKE.META.IGNORE_PATHS to ignore mtime of makefiles - which do
not matter in meta mode.