Refactor cleanup_procs_and_nuke

Stop flailing at the runsvdir process tree and shut it down properly.
If `initctl` doesn't properly shutdown the process tree then there is
a bug that needs to be identified and fixed.

I couldn't find any other use for the following methods so this commit
removes them completely.

graceful_kill
get_pgrp_from_pid
get_pids_from_pgrp
sigkill_pgrp

It also gets rid of the `kill_users` attribute.