kernel - Add lwkt.cache_threads boot-time tunable

kernel - Add lwkt.cache_threads boot-time tunable

* Add lwkt.cache_threads boot-time tunable.  This has to be set in
  /boot/loader.conf and controls how large a thread+kstack cache the
  kernel should maintain.

* Increase the default thread+kstack cache from 6 to 32 threads.

* Change the objcache_create*() API a little.  Instead of passing a
  mag_capacity field pass a nom_cache (nominal free objects to keep cached)
  field.

* Change the mag_capacity calculation to take into account small caches
  (the exec args cache) and remove the too-small-for-ncpus warning.

Related-to: IRC conversion thesjg & vsrinivas