Fix/improve nsclient.service

[Fantu]

Hi, while working on the packaging I saw a problem with the file service, the pid should not be in /usr but in /run also in some distros normally they have /usr in read-only and therefore it would not work either.

For Debian packaging for now I replace the upstream .service with an improved one (and probably will need other improvements:
https://github.com/Fantu/nscp/blob/agent-only-build4/debian/nsclient.service

In addition to the fixed PID, I added the network dependency.
I managed the logs within the service, instead of external creations and modifications (I only added log rotation external of the service in the package).

I still not tested the service using for now, I'm still working relating the build and initial package

[Fantu]

probably the pid can be also removed and systemd should already keep track without it on "Type=simple" if I'm not wrong