(rcirc-mangle-text): Add bold face property without replacing existing

(rcirc-mangle-text): Add bold face property without replacing existing
properties.
(rcirc-my-nick, rcirc-other-nick, rcirc-server)
(rcirc-nick-in-message, rcirc-prompt): Use min-colors and remove tty specs.
(rcirc-server-prefix, rcirc-server): New faces.
(rcirc-url-regexp): Generate with rx macro.
(rcirc-last-server-message-time): New variable.
(rcirc-filter): Record time of last message.
(rcirc-keepalive): Kill processes that did not send a message
since the last ping.
(rcirc-mode): Give rcirc-topic a local binding here.