systemctl: check if unit is masked in unit_find_paths()

systemctl: check if unit is masked in unit_find_paths()

When operating in client-side mode (force_client_side=true), unit_find_paths()
now checks if the unit file is masked (symlinked to /dev/null or empty) and
returns -ERFKILL, matching the behavior of the server-side path.

This centralizes masked unit detection in one place, making it consistent
across both client-side and server-side operations.