Pull out routines for interacting with the server and interpreting
internal commands which are not dependent on libreadline and
ncurses libraries.
This is a preparation step for a new lightweight birdc client.
(the current version UNIX-specific) anyway, so it's useless to try splitting it
to sysdep and generic part. Instead of this, configure script decides (based on
system type and user's wish) what (if any) client should be built and what
autoconfiguration it requires. Also, the client provides its own die/bug/...
functions.