* help.el (describe-mode): For clicks on mode-line, use "@"

* help.el (describe-mode): For clicks on mode-line, use "@"
interactive argument to get the major mode of the click instead of
the current buffer.

* isearch.el (isearch-mouse-2): Use new semantics of `key-binding'
in order to better redirect mouse-2 clicks.  Also allow default
bindings to apply.