Changed option types to `either str path` to allow using path values.
The related session variable is defined for the default and the extra
user directories now.
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
Before this change,
```rust
fn main() {
println!("{:?}", glib::get_user_special_dir(glib::UserDirectory::Documents));
}
```
would return `None` even though `~/Documents` is available and
`xdg.userDirs.enable = true`. Checking the differences between
`xdg-user-dirs-update` shows that the latter has quotes around each
thing.
PR #1440
