Implement malloc_domainset_aligned(9).

Change the power-of-two malloc zones to require alignment equal to the
size [*]. Current uma allocator already provides such alignment, so in
fact this change does not change anything except providing future-proof
setup.

Suggested by: markj [*]
Reviewed by: andrew, jah, markj
Tested by: pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28147