pgraph: Add fast pausing and exiting of graphs

This causes a graph to actually stop processing part way through, even
if there are poke's that want to continue on. This is so that the user
experience of pressing ^C actually causes a shutdown without finishing
the graph execution. It might be preferred to have this be a user
defined setting at some point in the future, such as if the user presses
^C twice. As well, we might want to implement an interrupt API so that
individual resource execution can be asked to bail out early if
requested. This could happen on a third ^C press.

test/shell/graph-exit1.sh

@@ -0,0 +1,10 @@
+#!/bin/bash -e
+
+# should take 15 seconds for longest resources plus startup time to shutdown
+# we don't want the ^C to allow the rest of the graph to continue executing!
+$timeout --kill-after=35s 25s ./mgmt run --yaml graph-exit.yaml --no-watch --no-pgp --tmp-prefix &
+pid=$!
+sleep 5s	# let the initial resources start to run...
+killall -SIGINT mgmt	# send ^C to exit mgmt
+wait $pid	# get exit status
+exit $?