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/service.c

@@ -1248,6 +1248,7 @@ int service_register(int type, char *cfg, struct rlimit rlimit[], char *file)
 		svc->noclear = tty.noclear;
 		svc->nowait  = tty.nowait;
 		svc->nologin = tty.nologin;
+		svc->notty   = tty.notty;
 
 		/* TTYs cannot be redirected */
 		log = NULL;