Add some convenience functions for manipulating hooks, in particular,
    *binding-hook*.

    (UPDATE-HOOK) removes a specific hook, or adds a
    named hook function with a given body.

    As examples of using (UPDATE-HOOK), two facilities are provided:

    - (UPDATE-SECOND-KEY-APP-BINDINGS) manages app-launch key-bindings
    under the second-key mode based on the template
    *SECOND-KEY-APP-BINDINGS*.

    - (CHANGE-MODIFIER) changes existing modifier bindings. The
    default MOD-1 (Alt) modifier conflicts with many applications; the
    MOD-4 (Winkey) modifier is often free. (CHANGE-MODIFIER) can help to
    change the modifier of default keybindings from MOD-1 to MOD-4.