dbus: mirror legacy runlevel edge cases in Manager1.SetRunlevel

The bus method called sm_runlevel() unconditionally.  In runlevel 0
and 6 that aborts an in-flight shutdown, which INIT_CMD_RUNLVL
refuses with a warning, and during bootstrap it switches immediately
where the legacy path defers via cfglevel to the end of runlevel S.

Port both.  A bad runlevel argument still returns InvalidArgs, where
the legacy protocol acks silently: a typed interface rejects garbage.

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

src/sm.c

@@ -94,6 +94,30 @@ static void shutdown_wdt_cb(uev_t *w, void *arg, int events)
 	do_shutdown(halt);
 }
 
+/*
+ * User-requested runlevel change, shared by the legacy API and the
+ * D-Bus SetRunlevel method.  Refused in runlevel 0/6, deferred via
+ * cfglevel during bootstrap.
+ */
+void sm_request_runlevel(int lvl)
+{
+	if (IS_RESERVED_RUNLEVEL(runlevel) && runlevel != INIT_LEVEL) {
+		warnx("Cannot abort runlevel 6/0.");
+		return;
+	}
+
+	if (lvl == 0)
+		halt = SHUT_OFF;
+	if (lvl == 6)
+		halt = SHUT_REBOOT;
+
+	/* Runlevel S: user requested change in next runlevel */
+	if (runlevel == INIT_LEVEL)
+		cfglevel = lvl;
+	else
+		sm_runlevel(lvl);
+}
+
 /*
  * Console input callback - handles Ctrl-C during bootstrap wait
  */