caddy-frontend: Stabilise proxy headers

On backend side headers are asserted in tests:

 * X-Forwarded-For
 * X-Forwarded-Proto
 * X-Forwarded-Port
 * Host

In order to pass cleanly X-Forwarded-For from the frontend to the backend,
it's passed as X-Forwarded-For-Real in case of cached slaves.

Noted problem with IPv6 endpoint was used, as in this case 6tunnel IP would
be used.