Support Mozilla and Galeon.

Support Mozilla and Galeon.
By default, find the installed browser whichever it is.
(browse-url-mozilla): New function.
(browse-url-mozilla-sentinel): New function.
(browse-url-galeon, browse-url-galeon-sentinel): New functions.
(browse-url-default-browser): New function.
(browse-url-process-environment): Use browse-url-browser-display.
(browse-url-browser-display): Renamed from browse-url-netscape-display.
(browse-url-mozilla-startup-arguments, browse-url-galeon-program)
(browse-url-galeon-arguments, browse-url-galeon-startup-arguments)
(browse-url-mozilla-program, browse-url-mozilla-arguments): New vars.
(browse-url-browser-function): New default, new options.