Fix a few issues in the pressure logic when the available buffers run low:

- make sure the buffer reclaiming loop in buf_get() actually does something
  but spin, if `backoffpages' is nonzero and all free queues have been drained.
- don't forget to set a poor man's condition variable to nonzero before
  tsleeping on it in bufadjust(), otherwise you'll never get woken up.
- don't be too greedy and reassing backoffpages a large amount immediately
  after bufadjust() has been called.

This fixes reproduceable hangs seen during heavy I/O (such as `make install'
of many large files, e.g. run in /usr/src/lib with NOMAN=) on systems with
a challenged number of pages (less than a few thousands, total).

Part of this is temporary bandaid until a better pressure logic is devised,
but it's solving an immediate problem. Been in snapshots for a solid month.

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