Changing "make clean" and "make cleandir"

["Simon J. Gerraty" <sjg%crufty.net@localhost>]

A couple of comments on this.

I generally like the idea of bsd.*.mk all just populating CLEAN*

I'm not so crazy about attempting to clean the src tree unless
explicitly requested.

Apart from the issues cited of read-only src trees, it is a waste of
time, when you never allow building without objdirs.

FWIW I also favor 'make destroy' (which rm -rf's the objdir) over 'make
clean*'.  Destroy is much faster, and more reliable/thorough, and since
if using auto.obj.mk the objdir will be auto created when next you run
make - no hassle.

--sjg