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

[Juri Linkov]

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.