add a special Provides: to all login manager packages

[Chris Lumens <clumens redhat com>]

Anaconda needs to know whether it should set the default runlevel to 3
or 5.  We do this by determining whether or not a package containing a
login manager has been installed.  Right now we just have a list of
packages that contain login managers.  Sometimes, we get bitten by that
- like when kdm moved to a new package, or now when we have a bug report
regarding slim (#485789).

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.

Thoughts?

- Chris