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.

/reviewed-on nexedi/re6stnet!13

re6st/plib.py

@@ -22,14 +22,14 @@ def openvpn(iface, encrypt, *args, **kw):
     logging.debug('%r', args)
     return utils.Popen(args, **kw)
 
-ovpn_link_mtu_dict = {'udp4': 1500, 'udp6': 1500}
+ovpn_link_mtu_dict = {'udp4': 1432, 'udp6': 1450}
 
 def server(iface, max_clients, dh_path, fd, port, proto, encrypt, *args, **kw):
     if proto == 'udp':
         proto = 'udp4'
     client_script = '%s %s' % (ovpn_server, fd)
     try:
-        args = ('--link-mtu', str(ovpn_link_mtu_dict[proto]),
+        args = ('--link-mtu', str(ovpn_link_mtu_dict[proto] + 93),
                 '--mtu-disc', 'yes') + args
     except KeyError:
         proto += '-server'