More fine-grained NUMA knobs

[John Baldwin]

Some of the I/O device affinity stuff such as bus_get_domain() (and the
bus_get_cpus() I have in review in D5519) are useful on their own right even
if the VM system is not doing NUMA-aware allocations.  I think it would be
useful to be able to enable these two "prongs" of NUMA awareness
independently.  To that end, I have a little strawman patch that adds two
new kernel options: VM_NUMA_ALLOC and DEVICE_NUMA.  I actually think it is
probably worth enabling DEVICE_NUMA by default on x86 (and bumping the
default MAXMEMDOM to, say, 8 (quad-socket haswell)).

You can see the simple patch at:

https://github.com/bsdjhb/freebsd/compare/master...bsdjhb:numa_opts

Thoughts?

-- 
John Baldwin