The previous default /run/gunicorn.ctl requires root permissions.
Now uses $XDG_RUNTIME_DIR/gunicorn.ctl if available, otherwise
$HOME/.gunicorn/gunicorn.ctl. This works on Linux, FreeBSD, OpenBSD,
and macOS without requiring elevated privileges.
- Add _get_default_control_socket() helper in config.py
- Create parent directory automatically with 0o700 permissions
- Update gunicornc CLI to use the same default path
- Add unit tests for path selection and directory creation