XDG feature

[jw3]

Move XDG functionality behind a cargo feature that disables it by default.

The intent here is to remove XDG support from deployment, while keeping it as an option for development environments.

Without XDG enabled the following are used:

• config_dir: /etc/fapolicy-analyzer
• data_dir: /var/lib/fapolicy-analyzer
• log_dir: /var/log/fapolicy-analyzer

To use XDG in a development environment ensure the FEATURES file is present in the root of the workspace and add a line for xdg to it. This file is gitignored, which makes it the most convenient way of configuring XDG per workspace.

Closes #900

[jw3]

Supports #875

[jw3]

Behavior exhibited after rpm install is that the /etc/fapolicy-analyzer dir will be implicitly created and used for configuration, while the logs and data will remain in XDG land. This is because the Python side is not updated in this PR, and the data and logs are invoked from Python.

The remainder will be resolved in #875 or another supporting PR.

Post #875 there will be no need for the implicit creation of /etc/fapolicy-analyzer as it will be managed by the rpm at that point.