libbb: consolidate the code to set termios unbuffered mode

libbb: consolidate the code to set termios unbuffered mode

function                                             old     new   delta
set_termios_to_raw                                     -     116    +116
count_lines                                           72      74      +2
powertop_main                                       1458    1430     -28
top_main                                             943     914     -29
more_main                                            759     714     -45
fsck_minix_main                                     2969    2921     -48
conspy_main                                         1197    1135     -62
rawmode                                               99      36     -63
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/6 up/down: 118/-275)         Total: -157 bytes

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