dsched - expand framework to track threads

dsched - expand framework to track threads

* The dsched framework now takes care of tracking threads/procs and
  bufs. Most of this code was factored out of dsched_fq.

* fq now uses the new, much simplified API, reducing the lines of code
  by about 50%.

* this will also allow for runtime policy switching, even to other
  policies that need to track procs/threads. Previously it was only
  possible to have one policy that tracked threads.

* Now all policies can be loaded at any time and will still be able to
  track all the threads.

* dsched_fq is now a module that can be loaded if required. Once loaded
  the policy is registered and ready to use with any disk.

* There is also a kernel option DSCHED_FQ now; otherwise
  dsched_fq_load="YES" has to be set in loader.conf to be able to use fq
  from boot on.

* Make a dsched sysctl tree.

Suggested-by: Aggelos Economopoulos