* src/ttt-client.c: (_ttt_client_execute_helo),
(_ttt_client_execute_quit), (_ttt_client_init), (_ttt_client_fini),
(_execute_request), (_handle_requests_thread): Implement QUIT. Add
client->registered flag and unregister only if registered. Change
from args_min and args_max to args_required.
* src/ttt-error.h:
* src/ttt-error.c: (ttt_error_string):
* src/ttt-server.c: (ttt_server_register_client): Put underscores
in TTT_ERRROR names for better readability.