[AC] help synced with FSF and removale of button-2

Mats Lidell writes:

 >  Additional changes
 >  - Remove use of button2 completely for following links
 >  - Use activate-function in extents so that the general link support
 >    for button1 kicks in and provides the required link functionality
 >  - Local implementation so that return does the same as button1 click
 >    (Generalizing this is beyond what I can manage and why do it until
 >    we agree it is a good thing!?)
 >  - Jump between "activate-function"-extents with press of the tab key

You've also deleted some functions without saying so, and without
saying why.  That's a no-no, somebody might have code in init.el that
uses them.

 > --- a/tests/automated/keymap-tests.el	Mon Jan 27 17:52:33 2014 +0100
 > +++ b/tests/automated/keymap-tests.el	Sun Mar 09 00:36:29 2014 +0100
 > @@ -36,7 +36,7 @@
 >                              find-function-at-point Q help-mode-bury button2
 >                              help-mouse-find-source-or-track p
                                                A
Have you run the tests?  Doesn't look like it --+