vi: use vsnprintf to format status line

vi: use vsnprintf to format status line

This is the last use of "vsprintf" in busybox:

function                                             old     new   delta
status_line_bold                                      72      77      +5
status_line                                           40      45      +5
vsprintf                                              23       -     -23
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 2/0 up/down: 10/-23)            Total: -13 bytes

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