If another ACME client is trying to solve a challenge for a name not
being served by Caddy on the same machine where Caddy is running, the
HTTP challenge will be consumed by Caddy rather than allowing the owner
to use the Caddyfile to proxy the challenge.

With this change, we only consume requests for HTTP challenges for
hostnames that we recognize. Before doing the challenge, we add the
name to a set, and when seeing if we should proxy the challenge, we
first check the path of course to see if it is an HTTP challenge;
if it is, we then check that set to see if the hostname is in the
set. Only if it is, do we consume it.

Otherwise, the request is treated like any other, allowing the owner
to configure a proxy for such requests to another ACME client.