Depend more on rcctl if it is present.

* Make the module support enable/disable of special services like pf via rcctl.
  Idea and method from @jarmani.
* Make the module handle when the user supplied 'arguments' variable does not
  match the current flags in rc.conf.local.
* Update description now that the code tries to use rcctl for everything if it
  is available.
Merge pull request #3 from eest/jarmani-openbsd_rcctl

Depend more on rcctl if it is present.