Fix #352: separate runlevel S from runlevel 0

Due to an old design decision runlevel S was encoded internally as '0',
meaning it was the same as halt/poweroff.  If you want to run scripts at
system shutdown this was less then ideal since it meant your scripts
also ran at bootstrap.

This change is quite invasive.  It introduces INIT_LEVEL (10) as the
value for runlevel S, meaning all code that parses and/or evaluates
anything for runlevel 0/S was affected and had to be reworked.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/svc.c

@@ -594,7 +594,7 @@ void svc_clean_dynamic(void (*cb)(svc_t *))
  */
 int svc_clean_bootstrap(svc_t *svc)
 {
-	if (!ISOTHER(svc->runlevels, 0)) {
+	if (!ISOTHER(svc->runlevels, INIT_LEVEL)) {
 		svc->pid = 0;
 		svc_del(svc);
 		return 1;