Create a cpuset mask for each NUMA domain that is available in the

Create a cpuset mask for each NUMA domain that is available in the
kernel via the global cpuset_domain[] array. To export these to userland,
add a CPU_WHICH_DOMAIN level that can be used to fetch the mask for a
specific domain. Add a -d flag to cpuset(1) that can be used to fetch
the mask for a given domain.

Differential Revision:	https://reviews.freebsd.org/D1232
Submitted by:	jeff (kernel bits)
Reviewed by:	adrian, jeff