use/x11/dm allows to switch between display managers
and cleans up the common bits of configuration too.

It's missing in sisyphus (and the last maintainer
was desperate with efforts required to keep it going),
just drop the cruft; if it ever returns, we'll revert this.

The problem at hand was that use/x11/xorg has been final,
and zerg@ just couldn't switch from nouveau to nvidia
when kdesktop needs that one.

Initial approach included a "big" FREE/PROP switch that
chose the particular KMODULES/PACKAGES to get added to
THE_* but that fails to achieve e.g. nvidia+radeon combo;
looks like these need individual switches.

This has had several goals:
- a target suitable for x86 and armh providing a rather
  minimal set of base xorg packages and generic drivers;
- task-oriented targets for graphics use cases:
  + "desktop" means rather 2D focus with 3D being welcome
    or even essential but not performance critical, thus
    "a slower driver is fine as long as it does work";
  + "3d" means specific 3D performance being critical,
    that is "no 3D means no use at all".

Regarding the free and proprietary 3D-capable drivers:
the previous idea was to split out some common ground
and then add the contenders on top of that; the current
approach is based on the observation that the live images
requiring proprietary NVIDIA/AMD drivers *by default*
are usually of not much use with hardware that lacks
proper 3D acceleration (like Tseng cards) or the driver
support for that (like Matrox these days).

Intel videodriver makes for a special case though:
it is both free and top-notch performer.

Thanks sem@ and boyarsh@ for discussion.

PS: xorg-drv-{keyboard,mouse,void} dropped;
those who need these can usually help themselves.

Status updates and readability tweaks all over the place.

(these *really* should be added atomically with the changes!)