first time.
share/mk/bsd.kmodule.mk: only set _INST_DIRS if KMODULEDIR is not
set. That we we avoid installing some bogus directories in
the DESTDIR and in $DESTDIR/METALOG.
tests/modules/k_helper/Makefile: add missing .include <bsd.own.mk>,
and set the KMODULEDIR that the author seems to have intended.