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

[Juri Linkov <juri@jurta.org>]

>> 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.")?

IIUC, the plan is to deprecate this feature, to not maintain two parallel
systems of user customizable variables.

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

With the proposed change above, users can continue to set variables
using `set-variable'.