Improvements to systemd service file as per #430.

Submitted by @rpodgorny.

contrib/iperf3.service

@@ -1,9 +1,10 @@
 [Unit]
 Description=iperf3
-Requires=network-online.target
+Requires=network.target
 
 [Service]
 ExecStart=/usr/bin/iperf3 -s
+Restart=on-failure
 
 [Install]
 WantedBy=multi-user.target