lorri needs to be able to write to /run/user/uid for the socket, to its
own cache directory ~/.cache/lorri and to the directory for gc roots.
(cherry picked from commit f06edaf18b)
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
