Add support for building modules with configured kernel options,

Add support for building modules with configured kernel options,
iff building modules with kernel.  Building modules standalone
(cd $kmoddir && make) is not affected.

This introduces an environment variable ``BUILDING_WITH_KERNEL''
which gets set to the kernel compile dir by its Makefile.
Required opt_*.h and use_*.h headers get copied from there to the
module build dir by kmod.mk.

While changing module Makefiles, clean up (slightly) incorrect
uses of kldload and kldunload (which are there for testing
purposes).

Reviewed-by: joerg