bug#67008: 30.0.50; Multiple major mode parents

[Stefan Monnier]

> I think it would help with certain customizations which check whether a mode
> is derived from e.g. js-mode.
> But whatever code that runs in parent-mode-hook won't run in the "derived
> modes" that use the new mechanism, would it?

The new functionality only affects `derived-mode-p` checks.
For mode hooks, it's up to the mode to run extra hooks if it deems them 
desirable.


        Stefan