systemd/eturnal: Use ProtectSystem=strict

[`ProtectSystem=strict` mounts more directories as read-only.][1]

[The upstream service unit only sets it to `full` to support older
systemd releases.][2] systemd 242, currently shipped by MarIuX, support
`strict`, so use it.

[1]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#ProtectSystem=
[2]: https://github.com/processone/eturnal/issues/14#issuecomment-843021648

misc_systemd_units/eturnal.service

@@ -18,6 +18,6 @@ RestartSec=3
 WatchdogSec=30
 LimitNOFILE=50000
 RuntimeDirectory=eturnal
-ProtectSystem=full
+ProtectSystem=strict
 NoNewPrivileges=true
 AmbientCapabilities=CAP_NET_BIND_SERVICE