shows only parts of larger outputs (for example 'show route all').
It seems that birdc reads (from bird) and writes (to stdout)
everything but during execution of some readline code some already
written output disappeared (although it is fflush()ed and
tcdrain()ed).

As birdc reads from stdin when select said there are some data,
O_NONBLOCK for stdin is unnecessary and when it is removed,
i didn't notified this problem.