Refactor catv. Move visible() from stty to libbb.

Refactor catv. Move visible() from stty to libbb.

Fixes the following TODO:

  stty's visible() function and catv's guts are identical. Merge them into
  an appropriate libbb function.

Also makes catv behave exactly like coreutils' cat -v e.g. it'll print 'M-^I'
instead of 'M-   '.

function                                             old     new   delta
visible                                                -      70     +70
do_display                                           431     379     -52
catv_main                                            306     250     -56
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/2 up/down: 70/-108)           Total: -38 bytes

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>