Sydent could give more helpful warnings if the Host header doesn't match the configured value

[reivilibre]

Right now, if Sydent is contacted by a homeserver as sydent.xyz.xyz but Sydent is configured as xyz.xyz, the request will fail by complaining about a corrupt or forged signature (SignatureVerifyException). It's fairly obscure.
This could be a lot better reported by making use of the HTTP Host header and detecting when that doesn't match what we expect.

Would have spotted #512 much more easily.

Potential problem: reverse proxies may not set the Host header by default. We could ameliorate this by providing example Nginx/... config that sets it, or warning if it's not present.