Updated systemd unit file and added sample configuration file

Rewrote the systemd unit file to fix an issue where the system was not able to create a PID file inside /var/run.

I fixed this by having systemd create a new directory called /var/run/gns3. Then I had systemd change ownership of the directory to gns3:gns3 so the gns3server executable could read and write the PID file. I have tested these changes against Ubuntu 16.04.1 LTS.

init/gns3.service.systemd

@@ -1,14 +1,16 @@
-[Unit]
-Description=GNS3 server
-
-[Service]
-Type=forking
-Environment=statedir=/var/cache/gns3
-PIDFile=/var/run/gns3.pid
-ExecStart=/usr/local/bin/gns3server --log /var/log/gns3.log \
- --pid /var/run/gns3.pid --daemon
-Restart=on-abort
-User=gns3
-
-[Install]
-WantedBy=multi-user.target
+[Unit]
+Description=GNS3 server
+
+[Service]
+Type=forking
+User=gns3
+Group=gns3
+PermissionsStartOnly=true
+ExecStartPre=/bin/mkdir /var/run/gns3
+ExecStartPre=/bin/chown -R gns3:gns3 /var/run/gns3/
+ExecStart=/usr/share/gns3/gns3-server/bin/gns3server --log /var/log/gns3/gns3.log --pid /var/run/gns3/gns3.pid --daemon
+Restart=on-abort
+PIDFile=/var/run/gns3/gns3-server.pid
+
+[Install]
+WantedBy=multi-user.target