Implement NUMA policy for kmem_*(9). This maintains compatibility with
reservations by giving each memory domain its own KVA space in vmem that
is naturally aligned on superpage boundaries.

Reviewed by: alc, markj, kib (some objections)
Sponsored by: Netflix, Dell/EMC Isilon
Tested by; pho
Differential Revision: https://reviews.freebsd.org/D13289