(gomoku-buffer-name): New constant.

(gomoku-buffer-name): New constant.
(gomoku-mode): Doc fix.  Add call to kill-all-local-variables,
for global-font-lock-mode.
(gomoku): Do not call gomoku-switch-to-window directly, else the
player is asked the startup questions twice after a killed game.
(gomoku-prompt-for-other-game): Throw an error if no new game,
since otherwise answering no after a killed game has no effect.
(gomoku-switch-to-window): Use gomoku-buffer-name.