res_musiconhold: Add kill_escalation_delay, kill_method to class

res_musiconhold:  Add kill_escalation_delay, kill_method to class

By default, when res_musiconhold reloads or unloads, it sends a HUP
signal to custom applications (and all descendants), waits 100ms,
then sends a TERM signal, waits 100ms, then finally sends a KILL
signal.  An application which is interacting with an external
device and/or spawns children of its own may not be able to exit
cleanly in the default times, expecially if sent a KILL signal, or
if it's children are getting signals directly from
res_musiconhoild.

* To allow extra time, the 'kill_escalation_delay'
  class option can be used to set the number of milliseconds
  res_musiconhold waits before escalating kill signals, with the
  default being the current 100ms.

* To control to whom the signals are sent, the "kill_method" class
  option can be set to "process_group" (the default, existing
  behavior), which sends signals to the application and its
  descendants directly, or "process" which sends signals only to the
  application itself.

Change-Id: Iff70a1a9405685a9021a68416830c0db5158603b