Refactor fallback shell handling, run as regular service

This patch re-enables the fallback shell handling after the big TTY
refactor.  We do this by allowing the fallback shell to run as a
regular service.

Note: this also adds the "hidden" support for 'notty' option for
      tty configurations stanzas.  This is just to pick up from
      where the kernel left us, reusing stdin + stdout.

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

src/svc.c

@@ -431,6 +431,10 @@ svc_t *svc_find_by_tty(char *dev)
 {
 	svc_t *svc, *iter = NULL;
 
+	/* this is valid for fallback shells */
+	if (!dev)
+		return NULL;
+
 	for (svc = svc_iterator(&iter, 1); svc; svc = svc_iterator(&iter, 0)) {
 		if (!svc_is_tty(svc))
 			continue;