This commit adds a new systemd service, `rclone-sync@.service`,
templated against the absolute path of a directory to keep in sync with
a (presumably) remote store.
Support for Backblaze B2 endpoints has been set up by default, but the
specific remote type can be configured via the `RCLONE_REMOTE_TYPE` host
variable. In addition, a default-passthrough remote that encrypts data
against a static password and salt has been defined under the `crypt`
name, and can also be used as the `RCLONE_DEST` of choice.
