The help command triggered by '?' keeps the message in readline buffer,
so it could be edited. For noninteractive shell it leads to an unexpected
side effect that `echo <cmd> ? | birdc` executes the command <cmd> after
showing its help. Avoid this by clearing the readline buffer in such case.
The BIRD client code is restructured that most of the code (including
main function) is shared in client.c, while birdc.c and birdcl.c contain
just I/O-specific callbacks. This removes all duplicated code from
variant-specific files.
