`set-variable' should use :set

[Drew Adams]

`set-variable' is in most respects a shortcut for using Customize to change an
option's value (for the current session - it does not save the value).  For
example, it uses the proper `interactive' spec and forces user input to be of
the correct defcustom :type.

However, it does not respect a defcustom :set.  I'm thinking that it should, so
that using `set-variable' would indeed be a proper shortcut for setting an
option value.

Am I missing something?  Is there some fundamental reason why we do not want to
do this or we should not do it?  Or is it just that we have never bothered to do
it?

If there is no special reason why we don't do it, then I think we should.  A
user should not encounter surprises due to :set not being invoked when using
`set-variable' and thus (perhaps important) value-change actions not being
carried out.

If we cannot, or for some reason should not, make such an enhancement, shouldn't
we at least warn the user that :set will not be invoked?  And perhaps require
confirmation after such a warning, before just setting the value?

WDOT?