similar to the kernel memory allocator.

This simplifies NUMA allocation because the domain will be known at wait
time and races between failure and sleeping are eliminated.  This also
reduces boilerplate code and simplifies callers.

A wait primitive is supplied for uma zones for similar reasons.  This
eliminates some non-specific VM_WAIT calls in favor of more explicit
sleeps that may be satisfied without new pages.

Reviewed by:	alc, kib, markj
Tested by:	pho
Sponsored by:	Netflix, Dell/EMC Isilon