Restore @console handling and support for external getty

- Drop vhangup(), how did things ever work *with* this in?!
 - setsid() + TIOCSCTTY are best buds, see notty code
 - Allow storing any non-NULL string as tty->dev, expand in service_register()
 - Reorder parse_cmdline_args(), we need to expand @console to current dev
 - Fix tty arg parser, swapparoo for external getty
 - Refactor tty_atcon(), iterated over by service_register()
 - New arg format, need to translate for old run_getty2()

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

src/tty.h

@@ -45,7 +45,7 @@ struct tty {
 char	*tty_canonicalize (char *dev);
 
 int	 tty_isatcon      (char *dev);
-int	 tty_atcon        (char *buf, size_t len);
+char	*tty_atcon	  (void);
 
 int	 tty_parse_args   (char *cmdline, struct tty *tty);