a bunch of rules, define a clean{kmod,lib,prog} target with the rules,
and have both clean and cleandir depend on that. That eliminates a bug
where 'cleandir' in a directory which included e.g. bsd.prog.mk but which
also had subdirs would 'make clean' all the subdirs and then 'make cleandir'
all ofthe subdirs. It also allows Makefiles to add more dependencies
to 'clean' after inclusion of the make template.
If 'clean' is already defined, the behaviour is the same as it used to be.
mycroft