add a special Provides: to all login manager packages

[Bill Nottingham]

Ignacio Vazquez-Abrams (ivazqueznet at gmail.com) said: 
> On Tue, 2009-02-17 at 11:43 -0500, Bill Nottingham wrote:
> > As a corollary, something that would allow /etc/X11/prefdm to not always
> > be hand-edited for new options would be nice as well.
> 
> How about this one? Just create a /usr/share/xdisplaymanager/foo.desktop
> containing "Exec=/usr/bin/bar" and putting DISPLAYMANAGER=FOO
> in /etc/sysconfig/desktop will make it use that.

If DISPLAYMANAGER is set, we just exec that, so we don't need changes
for that. It's the fallbacks at the end that require manual editing.

Bill