stty: enable line discipline display and change code if __linix__

stty: enable line discipline display and change code if __linix__

function                                             old     new   delta
stty_main                                           1242    1276     +34
do_display                                           419     431     +12
display_speed                                         90      81      -9
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 46/-9)              Total: 37 bytes

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