Add symbol versioning for libkadm5clnt

In order to support plugins for kadmin that use libkadm5srv, the
libkadm5clnt library has to be versioned to avoid hijacking all
of the function calls that should go to the server library.  Omit
the _kadm5_ clients from the public interface, and version
everything else.

Signed-off-by: Love Hörnquist Åstrand <lha@h5l.org>