button-buffer-map should inherit from special-mode-map

[Sam Steingold]

IMO, button-buffer-map should inherit from special-mode-map because
whenever it is used, the map which inherits from it is immediately
subject to suppress-keymap.
If button-buffer-map inherits from special-mode-map, then this is done
automatically, and, additionally, some useful bindings are inherited.

I will commit a patch to this effect unless someone objects.

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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