subdirmk: main.mk: clear SUBDIRMK_MAKEFILES MAKEFILE_TEMPLATES

Otherwise, if we reenter main.mk (for exmaple, there are some reasons
why one might want to $(MAKE) a particular target), and someone
foolishly said `export', we inherit SUBDIRMK_MAKEFILES from our
previous self and end up reading everything twice.  That causes things
not to work very well.

While we're here, clear MAKEFILE_TEMPLATES too.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>