[Slurm] Start Cargo as a systemd user-level service

Cargo is now started as a systemd user-level service whenever a
job requests for an ad-hoc storage service.

The following scripts have been added:

- `cargo@.service`: The Cargo systemd service file. Installed in
  `$SYSTEMD_UNIT_DIRECTORY` for user-level services (typically
  `/usr/lib/systemd/user/`)
- `cargoctl`: Control script for the Cargo user-level service. Installed
  in `$CMAKE_INSTALL_DATADIR/scord/cargoctl` (typically `/usr/share/scord`).
  Used internally by `cargo@.service`.

The `scord_prolog.sh` and `scord_epilog.sh` scripts have been updated to use
the new scripts when configuring the environment for a job.