These both arise when multiple applications are consuming free pages at
a high rate (e.g., > 1GB/s), and the inactive queue contains plenty of
clean pages.

1. After completing an inactive queue scan, we might still be below the v_free_min threshold, in which case there are threads still sleeping in VM_WAIT.

2. vm_pageout_wanted may be set without the free queues lock held. This can lead to a race:
   • the page daemon checks vm_pageout_wanted in vm_pageout_worker() and sees that it's false. vm_pages_needed is false.
   • a thread sets vm_pageout_wanted without holding the queue lock
   • the page daemon goes to sleep thinking that it has no work to do
   • a thread enters VM_WAIT, sees that vm_pageout_wanted is true and thus does not wake up the page daemon
   • the page daemon sleeps for a full second before starting another scan, despite the fact that there are threads in VM_WAIT

Fix 1) by ensuring that we immediately start a new inactive queue scan
if v_free_count < v_free_min and threads are blocked in VM_WAIT.

Fix 2) by adding a new subroutine, pagedaemon_wait(), called from
vm_wait() and vm_waitpfault(). It wakes up the page daemon if either
vm_pageout_wanted or vm_pages_needed is false, and atomically sleeps
on v_free_count.

I used two methods to generate load on the page daemon.

The first is a program that mmaps a region of anonymous memory
two times the size of RAM. In a loop, it reads a byte from each
page in the range. It periodically moves recently touched ranges
to the inactive queue with madvise(MADV_DONTNEED) to ensure
that the page daemon doesn't need to scan the active queue.
Three instances of this program are enough to bring the page daemon
to 100% CPU.

The second involves using truncate(1) to create a large sparse file,
and reading it back in a loop using dd(1).

Without this change, with both tests, page daemon throughput drops
off a cliff when multiple instances of the test loop are running, as a
result of the page daemon going to sleep for 1s while the test loops
are already blocked in VM_WAIT.