* riece-server.el (riece-server-keyword-map): Add :coding-system-alist.

* riece-server.el (riece-server-keyword-map): Add :coding-system-alist.

* riece-irc.el (riece-irc-open-server): Init riece-coding-system-alist.

* riece-handle.el (riece-handle-privmsg-message): Retry with the
channel's coding-system if it differs from the server's
coding-system.

* riece-commands.el (riece-command-send-message): Pass prefix to
riece-send-string.

* riece-globals.el (riece-coding-system-alist): New variable.

* riece-coding.el (riece-decode-coding-string-1): Split from
riece-decode-coding-string; set riece-coding-encoded-string and
riece-coding-decoded-coding-system properties on decoded string.
(riece-retry-decode-coding-string): New function.
(riece-decoded-coding-system): Use it.