resources: exec: Avoid possible deadlock race

Some of the early code I wrote probably wouldn't pass my own reviews
today. Here's one example of a rare deadlock that could sometimes occur
when a Process/CheckApply caused a shutdown, but the bufio tried to send
on a channel that nobody was going to read any more. Now we properly
unblock that send with a context.

test/shell/exec-fail.sh

@@ -6,7 +6,7 @@
 # TODO: should we return a different exit code if the resources fail?
 # TODO: should we be converged if one of the resources has permanently failed?
 $TIMEOUT "$MGMT" run --converged-timeout=15 --no-watch --no-pgp --tmp-prefix yaml --yaml exec-fail.yaml &
-
+# there's no ^C sent... it should shutdown on its own
 pid=$!
 wait $pid	# get exit status
 exit $?