emacs: help: remove duplicate bindings

If the user (or a mode) overrides a keybinding from the common keymap
in one of the modes then both help lines appear in the help screen
even though only one of them is applicable.

Fix this by checking if we already have that key binding. We do this
by constructing an list of (key . docstring) pairs so it is easy to
check if we have already had that binding. Then the actual print help
routine changes these pairs into strings "key \t docstring"