bug#67005: 30.0.50; improve nadivce/comp/trampoline handling

[Stefan Monnier]

>>> BTW, why is it a `defcustom` rather than a `defvar`?
>>> Do we really expect "normal users" to change it?
>> Yes, we do.  I think they actually did already, maybe Andrea remembers
>> which functions and why.
> I remember as well they did it, but can't remember precisely the reason.
> Actually I suspect it might have been before we had the trampoline
> system up and running.  ATM I can't picture any reason why a user would
> like to use this customize other than improving a specific backtrack or
> preventing a certain trampoline to be created, but indeed I might miss
> something.

Part of the question is also: would those users benefit from using
Custom to set this var, instead of using a plain `add-to-list` or
`setq`?


        Stefan