Change the way the way things that have to call the scheduler interact

Change the way the way things that have to call the scheduler interact
with it:

 * Return a unique integer id from scheduler.register(), to be passed back
   into other scheduler methods.

 * Control handler unregistration with the handler's return value, like
   some other frameworks do for similar callbacks.

 * Add a SpawnProcess.reg_id attribute to store the id returned from
   scheduler.register()

 * Pass the SpawnProcess.reg_id value into scheduler.schedule() calls,
   so the scheduler knows to return when the callback referred to by
   the given id unregisters itself by returning False.

svn path=/main/trunk/; revision=10921