Some pelab/plab event system hacks.

	* event/lib/event.h, event/lib/event.c: Add event_subscribe_auth
	which lets you specify whether any authentication should be done
	for events received through this subscription.

	* event/sched/event-sched.c: Handle EVPROXY objects.  Add a
	separate subscription for EVPROXY UPDATE events for each plab
	pnode (which might be too many...).  Also, need to update the EXPT
	field for events received through a noauth subscription so the
	proxies can figure out which experiments are active.

	* lib/libtb/tbdefs.h, lib/libtb/tbdefs.c: Add UPDATE event defs.

	* xmlrpc/emulabserver.py.in: Inject __plab_setup and
	__plab_teardown timelines into the eventlist when an experiment
	has plab nodes.  The __plab_setup timeline sends EVPROXY UPDATE
	events to each physical node while the __plab_teardown sends
	EVPROXY CLEAR events.  The __plab_setup timeline is run when the
	scheduler starts up, the __plab_teardown isn't run automatically
	yet.