watchpid_process(struct frame *fr)
Calls: timequeue_pid_frame, muf_event_add
Called by: prog_clean
When a process exits, it may be watching for another process's termination,
or another process may be watching for its termination.  This processes
all of that for the event system.
fr->watchees: Array of processes that we're watching for the termination
			  of.  We need to go through each of them, and clear the
			  entries in their watchers list.
fr->watchers: Array of processes that are watching for our termination.
			  We need to go through each of them, and send each of them
			  a PROC.EXIT event.  Once those events are sent, we clear
			  and free the structures in the relevant watchers and
			  watchees lists.

prog_clean(struct frame *fr)
Calls: watchpid_process, CLEAR, localvar_freeall, scopedvar_freeall,
		delete_seed, muf_dlog_purge, dequeue_timers, muf_event_purge
Called by: 
mufevent.c:	static muf_event_process_free, muf_event_dequeue,
			muf_event_process
timequeue.c:static free_timenode, add_event, handle_read_event(4),
			dequeue_prog_real
interp.c:	do_abort_loop, interp_loop(4)
This walks the frame and frees everything related to it -- all programs
that have been started in it, all fors, trys, var, lvars, scopedvars of
all of them, instructions, and everything.
FIXME: This explanation of how the frame works should go somewhere else.
The frame is set up as soon as a program is called.  Frames are
passed around by pointer, and always referred to as 'fr' wherever they
are.
fr->