ntopng: Fix a couple of bugs.

This fixes a couple of bugs reported by the AUR user pdifeo:
  - The systemd unit file was named incorrectly.
  - The joint script arguments were not being passed on.

From pdifeo's original comment:

> In the file ntopng.service there is
> ....
> ExecStart=/usr/bin/ntopng -i %I
>
> and in the file /usr/bin/ntopng
> #!/bin/bash
> cd /usr/share/ntopng
> ./ntopng
>
> If I understand, the %I parameter is never passed to executable.
> The first issue is on ntopng.service. The correct file name must be
> ntopng@.service. The second issue is that ./ntopng is called into
> script without $@ for passing parameters. so the file /usr/bin/ntopng
> should be
>
> #!/bin/bash
> cd /usr/share/ntopng
> ./ntopng $@
>
> With these changes the service start well with
> systemctl start ntopng@<interface name>
>
> Another issue is on compilation stage. Missing
>
> make geoip
>
> for geoip database creation.
>
> There is other little problems with data directories. I try to
> understand better.

The `make geoip` issue was fixed in a previous commit.