Clean up uma_int.h a bit.

Clean up uma_int.h a bit.

This makes it easier to write libkvm programs that access UMA data
structures.

- Remove a couple of unused slab functions and make others local to
  uma_core.c.  Similarly move SLAB_BITSETS, which affects the layout of
  slab structures, to uma_core.c.
- Stop defining the slab structures under _KERNEL.  There's no real
  reason they can't be visible to userspace like the rest of UMA's
  structures are.
- Group KEG_ASSERT_COLD with other keg macros.
- Convert an assertion about MAXMEMDOM to use _Static_assert.

No functional change intended.

Discussed with:	jeff
Reviewed by:	rlibby
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23980