sort: add -V "sort version"

sort: add -V "sort version"

function                                             old     new   delta
packed_usage                                       32855   32858      +3
get_key                                              500     503      +3
sort_opt_str                                          36      37      +1
sort_main                                           1037    1036      -1
compare_keys                                         795     783     -12
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 7/-13)              Total: -6 bytes

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