Restore interface ethtool settings and MTU on stop

When Zeek is stopped it doesn't restore the interface ethtool settings nor the MTU. It should ideally restore these, though a system reboot would work as well.

`ifconfig` to check the MTU
`ethtool -k <interfacename>` to check ethtool settings

https://packages.zeek.org/packages/view/ce1462d8-9348-11eb-81e7-0a598146b5c6