vi: discover window size even on serial consoles. optional

vi: discover window size even on serial consoles. optional

function                                             old     new   delta
edit_file                                            671     761     +90
wh_helper                                              -      57     +57
query_screen_dimensions                               54      63      +9
ar_main                                              533     542      +9
refresh                                              767     773      +6
vi_main                                              242     243      +1
text_yank                                             56      54      -2
get_terminal_width_height                            180     135     -45
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/2 up/down: 172/-47)           Total: 125 bytes

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