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
In particular, don't add trailing backslashes introduced by
`xautolockExtraOptions`. Systemd's unit file parser seems to have
gotten a bit stricter and with systemd 242, the trailing backslash
caused the next non-empty line to be ignored.
In that case, this was `[Section]`, so all subsequent settings were
mistakenly added to `[Service]`, causing them to be ignored entirely.
Simplify and fix this by using `concatStringsSep` to build a single
`ExecStart` line.
