Don't require jabber-console in defcustom of jabber-debug-log-xml

That makes it impossible to run Emacs if jabber-debug-log-xml has been
customized, but jabber-console.el is not in the load path for some
reason.  Instead add an autoload cookie for jabber-process-console to
ensure that it gets loaded when needed.  Also remove duplicate
definition of jabber-buffer-connection.