kbd_mode: try harder to find console device if -C TTY is not given

kbd_mode: try harder to find console device if -C TTY is not given

Was (under X):
	$ ./busybox_old kbd_mode
	kbd_mode: ioctl 0x4b44 failed: Inappropriate ioctl for device
Now:
	$ ./busybox kbd_mode
	The keyboard is in off mode

function                                             old     new   delta
kbd_mode_main                                        166     174      +8
packed_usage                                       31782   31764     -18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 8/-18)             Total: -10 bytes
   text	   data	    bss	    dec	    hex	filename
 915757	    485	   6880	 923122	  e15f2	busybox_old
 915747	    485	   6880	 923112	  e15e8	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>