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/api.c

@@ -375,7 +375,7 @@ static void api_cb(uev_t *w, void *arg, int events)
 		case INIT_CMD_HALT:
 		case INIT_CMD_POWEROFF:
 		case INIT_CMD_SUSPEND:
-			if (runlevel == 0 || runlevel == 6) {
+			if (IS_RESERVED_RUNLEVEL(runlevel)) {
 				warnx("Unsupported command in runlevel S and 6.");
 				goto leave;
 			}