add a special Provides: to all login manager packages

[clumens at redhat.com]

> Since hardcoded lists always suck, I'm therefore proposing that all
> packages containing a login manager include a special Provides: that we
> can query on.  Then nobody ever has to come asking for a change again,
> and if package names change, it's fine.

After looking at everyone's suggestions and thinking a bit on my own, I
think any of the following are acceptable to me:

   Provides: login(graphical)

   Provides: config(graphical-login)

   Provides: service(graphical-login)

I like making it clear that this is a graphical login, since I'd hate
for the "login" program to provide this as well.  service() makes some
sense since gdm - at least - continues to run after you've logged in.
That's kind of service-like.

Besides that, I don't care.  Any opinions?  If we can arrive at some
consensus, I'll start filing bugs against packages to get this set up.

- Chris