sending any message to the monitor, the sender must check a
"ready" channel.  Before exiting, the monitor records its exit
error and closes this channel, ensuring that all later reads
from the ready channel will immediately return false.

Inspired by
http://chplib.wordpress.com/2009/09/30/poison-concurrent-termination/

R=rsc
APPROVED=rsc
DELTA=47  (27 added, 11 deleted, 9 changed)
OCL=35782
CL=35784