MTU on wireguard-apple for IPv6

[Rémi Lapeyre]

> Probably the phone's dhcp server doesn't pass the right MTU back to
> the client, but then the phone does the mss-mtu hack to fix up TCP
> connections. That hack doesn't work when the traffic is end-to-end
> encrypted, as is the case with WireGuard, so no smaller MSS is ever
> negotiated.

Won’t this situation happen a lot? Should we fix the default MTU to 1280
to avoid users having their tunnel randomly fail on some network?