Fallback rescue mode handling

Before 4e5d06b the only way to go into rescue mode was to skip certain
steps and then attempt to load /lib/finit/rescue.conf.  After 4e5d06b
we keep the old handling as a fallback in case early sulogin fails.

The new tty option 'rescue' is added, which recue.conf is updated with.
This option implies notty mode and will try sulogin (again) before it
falls back to start /bin/sh as a login shell.

The reason we keep this is to be able to handle all possible use-cases
and also allow sysadmins to set up the behavior that fits their needs.

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

src/tty.h

@@ -40,6 +40,7 @@ struct tty {
 	char	 nowait;
 	char	 nologin;
 	char	 notty;
+	char	 rescue;
 };
 
 char	*tty_canonicalize (char *dev);