Rename FH_BAD to FH_NADA throughout.

* devices.h (FH_ERROR): New value.
(iscons_dev): Extend to detect all the console device types.
* devices.in: Set aside storage for FH_ERROR.
* dtable.cc (dtable::init_std_file_from_handle): Use iscons_dev to detect when
device is a console.
(fh_alloc): Pass device to console constructor.
(build_fh_pc): Short circuit when we detect that the constructor saw an error.
* fhandler.h (fhandler_console::fhandler_console): Accept fh_devices parameter.
(get_tty_stuff): Change to void.
* fhandler_console (fhandler_console::set_unit): Set device to FH_ERROR on
attempt to access anything other than the current console.
(fhandler_console::get_tty_stuff): Change to void return.
(fhandler_console::open): Return EPERM on FH_ERROR device type.
(fhandler_console::fhandler_console): Set the device type appropriately before
calling get_tty_stuff and rely on that function to reset it if necessary.

winsup/cygwin/fhandler_tty.cc

@@ -472,7 +472,7 @@ process_ioctl (void *)
 fhandler_tty_slave::fhandler_tty_slave (int unit)
   : fhandler_tty_common (), inuse (NULL)
 {
-  if (unit >= 0)
+  if (unit > 0)
     dev ().parse (DEV_TTYS_MAJOR, unit);
 }