add pre and post state change exectables

The pre state change executable can for example check if a service status
is good before a status switch.  Alternatively some additional action,
such as iptables rule changes can take place to block or allow multicast
traffic depending on whether a service state is disabled or enabled.
Only the imagination is a limit what could be done at state changes.

Proposed-by: Warren Church <wchurch@lastminute.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>