Add two new kernel options to control memory locality on NUMA hardware.

• UMA_XDOMAIN enables an additional per-cpu bucket for freed memory that was freed on a different domain from where it was allocated. This is only used for UMA_ZONE_NUMA (first-touch) zones.
• UMA_FIRSTTOUCH sets the default UMA policy to be first-touch for all zones. This tries to maintain locality for kernel memory.

Reviewed by: gallatin, alc, kib
Tested by: pho, gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20929