sleep: fix "sleep -- ARGS"

function                                             old     new   delta
sleep_main                                           116     119      +3
printf_main                                          860     837     -23
single_argv                                           50      25     -25
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-48)             Total: -45 bytes

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