bug#10773: set-variable can't change values of user options

[Drew Adams]

> In my opinion, the best way to eliminate the distinction
> between user variables (with the leading '*' in the docstring)
> and customizable variables in 24.2 is:
> 
> 1. Make `user-variable-p' an alias for `custom-variable-p'.
> 
> 2. Remove the `user-variable-p' condition from `set-variable'.
> 
> This will fix the reported case where `set-variable' can change
> the value of `outline-mode-hook', but not `outline-minor-mode-hook';
> and many other similar cases.

And how will that help code that uses
(defvar foo "*A user variable.")?

Users have always been able to set such a user variable using `set-variable'.