cron: add procd listeners for crontabs

Add procd file listeners to check files in `/etc/crontabs/`.

Also unified a bit the function style.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit cbf69fb2ad)

package/utils/busybox/files/cron

@@ -29,10 +29,12 @@ start_service () {
 
 	procd_open_instance
 	procd_set_param command "$PROG" -f -c /etc/crontabs -l "${loglevel:-5}"
+	for crontab in /etc/crontabs/*; do
+		 procd_set_param file "$crontab"
+	done
 	procd_close_instance
 }
 
-service_triggers()
-{
+service_triggers() {
 	procd_add_validation validate_cron_section
 }