Fix 'make installcheck' to check for missing UID/GID as well, broken
since r249893, by adding a separate _installcheck_world and
_installcheck_kernel so the destination targets can be more explicit
on which they are needed for.

installcheck will call both, while installworld only calls
_installcheck_world and installkernel only calls _installcheck_kernel

While here, mark the internal targets as starting with _.

Reported by:	des
Reviewed by:	des
Pointyhat to:	bdrewery
Approved by:	re (delphij)