vi: code shrink, proper printf formatting for strlen()

vi: code shrink, proper printf formatting for strlen()

function                                             old     new   delta
yank_delete                                           98      99      +1
what_reg                                              34      33      -1
text_yank                                             56      54      -2
end_cmd_q                                             17      14      -3
do_cmd                                              4718    4705     -13
colon                                               2875    2861     -14
edit_file                                            668     648     -20
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/6 up/down: 1/-53)             Total: -52 bytes

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