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