displaymanager: Various fixes and cleanups.

* Fix typos in displaymanager.conf comments and make the settings follow the naming convention..
* Don't error if a DM is not installed, just print a debugging message and proceed.
* Remove lightdm gdmflexiserver hack (not portable, not applicable to current versions of LightDM, not necessary for older ones except possibly very ancient ones).
* Fix default session setup for GDM. (This has to be done per-user, so we can only do it in the autologin setup. Doing it for the gdm user has no practical effect.)
* Use libcalamares.utils.debug instead of print to print a debugging message during SDDM autologin setup.
* Disable basic setup (best left to distribution packages) by default, allow enabling it through displaymanager.conf.
* Fix the use of getent in the basic setup.
* Don't run xdg-icon-resource and update-desktop-database during KDM setup (not related to KDM, or even to display managers in general).
* Also look for display managers in /usr/sbin.