shutdown: limit wait time if forced

Problem: the 'flux queue idle' command in the shutdown script
can wait a long time if jobs are stuck in CLEANUP.

If the script receives the -f option from 'flux shutdown --force',
abort after a 10s timeout.

etc/shutdown

@@ -1,5 +1,16 @@
 #!/bin/sh
 
+force=
+while getopts f namer; do
+    case $name in
+    f) force=1 ;;
+    esac
+done
+
 flux queue stop --quiet --all --nocheckpoint
 flux cancel --user=all --quiet --states RUN
-flux queue idle --quiet
+
+if $force; then
+    timeout="--timeout=10s"
+fi
+flux queue idle --quiet $timeout