shuf: improve help text

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
shuf: do not use strings for -i RANGE case

function                                             old     new   delta
shuf_main                                            482     496     +14

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
shuf: trim help text

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