Change the zone setup:
- Allow slabs to be returned to the OS
- Set the number of slots to the max devctl will queue before discarding
- Reserve 2% of the max (capped at 100) for low memory allocations
- Disable per-cpu caching since we don't need it and we avoid some pathologies

Change the alloation strategiy a bit:
- If a normal allocation fails, try to get the reserve
- If a reserve allocation fails, re-use the oldest-queued entry for storage
- If there's a weird race/failure and nothing on the queue to steal, return NULL

This addresses two main issues in the old code:
- If devd had died, and we're generating a lot of messages, we have an
  unbounded leak. This new scheme avoids the issue that lead to this.
- The MPASS that was 'sure' the allocation couldn't have failed turned out
  to be wrong in some rare cases. The new code doesn't make this assumption.

Since we reserve only 2% of the space, we go from about 1MB of
allocation all the time to more like 50kB for the reserve.

Reviewed by: markj@
Differential Revision: https://reviews.freebsd.org/D26448