*: introduce and use ffulsh_all()

*: introduce and use ffulsh_all()

function                                             old     new   delta
buffer_fill_and_print                                179     196     +17
fflush_all                                             -       9      +9
spawn                                                 87      92      +5
rtcwake_main                                         455     453      -2
...
alarm_intr                                            93      84      -9
readcmd                                             1072    1062     -10
bb_ask                                               345     333     -12
more_main                                            845     832     -13
flush_stdout_stderr                                   42      23     -19
xfflush_stdout                                        27       -     -27
flush_stderr                                          30       -     -30
------------------------------------------------------------------------------
(add/remove: 1/2 grow/shrink: 2/50 up/down: 31/-397)         Total: -366 bytes

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