relaymsg should publish a full NUH

[slingamn]

The lack of a full NUH with RELAYMSG is confusing some clients. Let's add ~u as the username and a hostname, probably from ComputeAccountCloak against the relay nick (the separator should ensure that it doesn't collide with real account cloaks).

[slingamn]

I decided to send the relaying client's hostname as the hostname.