Add a lightdm.conf entry to set environment for xserver-command et al.

[Laércio de Sousa]

Currently, if you need a custom X server command with some environment variables previously set, you can do something like this in lihgtdm.conf:

xserver-command=env FOO=bar BAR=baz /usr/bin/X (...)

However, this approach is a little bit inconvenient if you only want to set environment, not customize the command line.

It would be handy if lightdm.conf gains a dedicated entry for setting the environment. I suggest the following variants (inspired by systemd service unit files):

xserver-environment=FOO=bar BAR=baz (sets variables directly)

xserver-environment-file=/etc/default/foo (provides a file that should be sourced in X server environment)

Of course, this idea could be extended to other display servers.