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.

/reviewed-on nexedi/re6stnet!13