Initial implementation of the client side GSSAPI authenticator. The
current version successfully authenticates clients using the same style as
the MariaDB GSSAPI plugin does. Currently, it is not possible to acutally
use the plugin as the backend server would require the matchig GSSAPI
plugin.
Also added skeleton code for the backend GSSAPI authenticator. It only
implements the required entry points and sends the client auth packet to
the backend.
