Add systemd service file

[Danilo Bargen]

Coturn provides a service file example here: https://github.com/coturn/coturn/blob/master/rpm/turnserver.service.fc Would be great if this could be added to the package to replace the init script.

[Danilo Bargen (gwrtheyrn)]

Here is an example file that seems to work fine:

```
[Unit]
Description=coturn
Documentation=man:coturn(1) man:turnadmin(1) man:turnserver(1)
After=syslog.target network.target

[Service]
Type=forking
User=turnserver
Group=turnserver
RuntimeDirectory=turnserver
RuntimeDirectoryMode=0750
EnvironmentFile=/etc/default/coturn
PIDFile=/run/turnserver/turnserver.pid
ExecStart=/usr/bin/turnserver --daemon --pidfile /run/turnserver/turnserver.pid --syslog -c /etc/turnserver.conf $EXTRA_OPTIONS
Restart=on-abort
LimitCORE=infinity
LimitNOFILE=1000000
LimitNPROC=60000
LimitRTPRIO=infinity
LimitRTTIME=7000000
CPUSchedulingPolicy=other
UMask=0007

[Install]
WantedBy=multi-user.target
```

To enable verbose logging, /etc/default/coturn should contain `EXTRA_OPTIONS=-v`.

To bind to port 443, the /usr/bin/turnserver binary needs the CAP_NET_BIND_SERVICE capability (`setcap /usr/bin/turnserver cap_net_bind_service=+ep`).

[pablo (pablo-platt)]

Danilo, is it possible to install the package, disable the init script and manually add this service file?
Maybe leaving TURNSERVER_ENABLED=0 in /etc/default/coturn is enough?

[Danilo Bargen (gwrtheyrn)]

Yes, it is possible to manually add a systemd unit file to the package, but it would be good if this were supported natively by the Ubuntu package.

[pablo (pablo-platt)]

Why does this unit file have differences compared to the rpm unit file?
https://github.com/coturn/coturn/blob/master/rpm/turnserver.service.fc

1. The PIDFile is under /run although both the rpm and the ubuntu init.d file use /var/run
2. ExecStart adds --pidfile in addition to PIDFile. Isn't this redundant?
3. ExecStart adds --syslog. why?
4. ExecStopPost is removed.
5. LimitNOFILE=1000000 compared to 999999

Thanks

[Danilo Bargen (gwrtheyrn)]

Hm, I don't actually remember where I based that file on, sorry... The difference are not deliberate, or at least not by me.