OpenVPN 2.4.7 workaround to revert to previous MTU settings

In commit 06974788, we increased the --link-mtu value as a temporary way to compensate the unexplained behaviour change of recent OpenVPN.

This was partly due to encryption, which was enabled despite --cipher none. And it happens that the behaviour of --link-mtu only changed for the server, with a mysterious difference of 93 bytes.

Hence the workaround to get identical tunnel MTU on both sides.