init: Add a "starting" state for services

 * We might want to trigger something before actually execing the
   service in question in order to do some pre-flight tasks.

 * This lets you do the following:

   on property:init.svc.awesomed=starting
     write /sys/module/awesomemod/parameters/everything_is_awesome 1

   ..and when awesomed starts, it will run this trigger before forking.

Change-Id: I9756b46eafd884d367fe8304b859de2152bdfeb4