UMA NUMA flag day. UMA_ZONE_NUMA was a source of confusion. Make the names
more consistent with other NUMA features as UMA_ZONE_FIRSTTOUCH and
UMA_ZONE_ROUNDROBIN. The system will now pick a select a default depending
on kernel configuration. API users need only specify one if they want to
override the default.

Remove the UMA_XDOMAIN and UMA_FIRSTTOUCH kernel options and key only off
of NUMA. XDOMAIN is now fast enough in all cases to enable whenever NUMA
is.

Reviewed by: markj
Discussed with: rlibby
Differential Revision: https://reviews.freebsd.org/D22831