add a3d_workqpurge_fn and return task status

Some use cases might want to be notified when a task is
purged from the workq and if it has completed.
tweak a3d_list_insert and a3d_list_append to accept null item

insert/append may be preferred over push/enqueue when a
listitem is needed
add a3d_list_move and a3d_list_moven
add a3d_cache

Note: initially created in 2010 as a texture cache.