Provide stronger way to declare UDP won't work

Right now there's no way for a node to declare in its Hostinfo/NetInfo that it absolutely is incapable of doing UDP so don't even try.

The Netinfo.UDP field is close, but it technically only says whether STUN UDP worked. Local LAN UDP might still work, so clients should try.

But `js/wasm` (#3157) can never do UDP, so even doing a CallMeMaybe disco message over DERP is pointless. Clients should rather just start using DERP directly without the disco work at all, once we have a way to declare that. (Without making assumptions from Hostinfo.OS).

/cc @danderson