* PROTOCOL: Removed unused servername

        * src/ttt-client.h: * src/ttt-client.c:
        (_ttt_client_execute_helo), (_ttt_client_init),
        (_ttt_client_fini), (_execute_request), (ttt_client_get_name),
        (ttt_client_set_name): Added command descriptions structure. Match
        command case-insensitively, dispatch to execute
        function. Implement HELO command. Register on HELO instead of on
        connect.

        * src/ttt-server.h: * src/ttt-server.c: (ttt_server_init),
        (ttt_server_register_client), (ttt_server_unregister_client),
        (ttt_server_get_host), (ttt_server_get_port), (main): Added host
        and port to server. Make client registration name-based rather
        than id-based.

        * src/x.c: (xstrdup): Return NULL on NULL

        * src/ttt-lex.h: Regenerated