In addition to any existing filesystem restrictions on the (www-data)
backup user these config options uses namespaces and other kernel
features to further restrict what the _rest-server_ is allowed to do.
* `ProtectSystem=strict` and `ReadWritePaths=/path/to/backups` ensures
that the _rest-server_ is only allowed to write to its data directory.
* `ProtectHome=yes` and `PrivateTmp=yes` limits what the _rest-server_
gets (read) access to.
* `NoNewPrivileges=yes` prevents the _rest-server_ from using setuid
binaries, etc to escalate its privileges.
See https://www.freedesktop.org/software/systemd/man/systemd.exec.html
for further details
While at I also replaced the _/tmp/restic_ path with a more explicit
placeholder path. Given that one rarely wants to backup to _/tmp_ I
figured it better to force a choice of path rather than to have
someone accidentally end up using _/tmp/restic_ for their backups.
The directive "StartLimitInterval" has been replaced by [StartLimitIntervalSec=interval, StartLimitBurst=burst](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#StartLimitIntervalSec=interval). I'd suggest that the default backoff settings are fine (in Ubuntu 19.10 no more than 5 restarts per 10 seconds, else delayed by 10 seconds per attempt) so this directive can simply be removed.
