Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
When running the service start script with `DISPLAY` set, a `gi`
import error is triggered. Blanking the variable will make the script
use a different code path that does not attempt to import `gi`.
Also moves activation script up into start of script instead.
PR #1415
