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

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