WIP: Add taskq and thread for driving low level kmem activities.
WIP: Release memory immediately if machine is under memory pressure.
WIP: add support for timeout/untimeout.
WIP: Bring in more Illumos kernel files to support its kmem.h
(avl, kmem_impl etc)
WIP: Add initial implementation of a magazine based memory cache.
     This is largely based on the Illmos implementation but
     is significantly cut down - just a basic implementation.

     At this stage some of the management tasks are not in place.

     NOTE: This has not been run in kernel yet.