Ensure that imports into per-domain kmem arenas are KVA_QUANTUM-aligned.

The old code appears to assume that vmem_alloc() would import
size-aligned KVA chunks from the parent kernel_arena, but vmem doesn't
provide this guarantee.

Also remove the unused global RWX arena and add comments explaining why
we have per-domain arenas.

Reported by: alc
Reviewed by: alc, kib (previous version)
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17249