Put client on a stony ground. The whole client is going to be system-specific

Put client on a stony ground. The whole client is going to be system-specific
(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.