systemd/eturnal: Let epmd only listen on localhost

From [issues #9 (*Document ERL_EPMD_ADDRESS and ERL_EPMD_PORT*)][1]:

> === ERL_EPMD_ADDRESS ===
>
> On eturnal startup, an Erlang Port Mapper Daemon (epmd) service is spawned (if
> not running already). This service listens on all available interfaces by
> default. Setting the `ERL_EPMD_ADDRESS' environment variable tells epmd to
> listen on the specified comma-seperated list of IP addresses instead. Note that
> the IPv4 and IPv6 loopback addresses are implicitly added to this list if not
> specified. It's usually recommended to set `ERL_EPMD_ADDRESS' to `127.0.0.1'
> (e.g., by calling `systemctl edit eturnal' and adding an
> `Environment="ERL_EPMD_ADDRESS=127.0.0.1"' line to the `[Service]' section).

[1]: https://github.com/processone/eturnal/issues/9

misc_systemd_units/eturnal.service

@@ -9,7 +9,7 @@ Documentation=https://github.com/processone/eturnal/blob/1.4.1/CHANGELOG.md
 [Service]
 Type=notify
 User=eturnprj
-Environment=ETURNAL_ETC_PREFIX=/project/eturnal
+Environment=ETURNAL_ETC_PREFIX=/project/eturnal ERL_EPMD_ADDRESS=127.0.0.1
 ExecStart=/project/eturnal/eturnal-current/bin/eturnalctl foreground
 ExecReload=/project/eturnal/eturnal-current/bin/eturnalctl reload
 ExecStop=/project/eturnal/eturnal-current/bin/eturnalctl stop