Provide facility to auto-add users to required groups for xorgxrdp

[matt335672]

See #1029 and #2841 for the background on this.

When running with Glamor, it's often necessary for the user to be added to a supplementary group to be able to open /dev/dri/renderD128.

My question is; should we be doing this for the X server process automatically? My thinking is adding a line to the [Xorg] section of sesman.ini like this:-

[Xorg]
SupplimentaryGroups=render
param=/usr/lib/xorgo/Xorg
. . .

and then in sesexec, when the X server is started we add in the required groups before dropping permissions.

The main advantage is the user is not explicitly added to a group that they might only be using to run Xorg.

Thoughts anyone? Is the [Xorg] section the right place to add this? It might be easier for the distros if it was added elsewhere maybe.