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