Make universe configuration more consistent with rest of system

Add 'WITHOUT_WORLDS' and 'WITHOUT_KERNELS' as aliases for the inconsistently
named MAKE_JUST_KERNELS and MAKE_JUST_WORLDS respectively. I always forget the
MAKE_ part (or is it BUILD_), and it's inconsistent with everything
else. Document the new things, but leave speculation of any eventual MAKE_JUST_*
deprecation out of the manuals and comments.

Reviewed by: brooks, bdrewery, emaste (LGTM)
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D24212