smxi: new feature - select default display login manager

[techAdmin]

For fresh installs without any current default display manager set, smxi package install, display manager install, will let you pick from a list: kdm gdm xdm wdm slim.

If you are installing kde, it will default automatically to use kdm, for non kde, it will default to wdm unless you override that default using the new display manager selector option (run prior to desktop install).

I'm basically improving this to enable users, including myself, to build up a system from scratch using debian business card install cd, making it exactly how they want it, but still with all the smxi ease of use.

This complements the new fluxbox installer, and the reworked basic package group for all lightweight desktops.

slim is not in stable, so that is not supported for stable installs.