Implement a VM load heuristic. sysctl vm.vm_load will return an indication

Implement a VM load heuristic.  sysctl vm.vm_load will return an indication
of the load on the VM system in the range 0-1000.

Implement a page allocation rate limit in vm_fault which is based on
vm_load, and enabled via vm.vm_load_enable (default on).  As the system
becomes more and more memory bound, those processes whos page faults
require a page allocation will start to allocate pages in smaller bursts
and with greater and greater enforced delays, up to 1/10 of a second.

Implement vm.vm_load_debug (for kernels with INVARIANTS), which outputs
the burst calculations to the console when enabled.

Increase the minimum guarenteed run time without swapping from 2 to 15
seconds.