Add support for Fowarded header (RFC 7239)

[nhairs]

We should probably support the Forwarded header which was standardised in RFC 7239.

One of the benefits of this header over X-Forwarded-* headers is that it supports including the port of the client rather than just the address.

See also: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Forwarded

[Kludex]

• I've created Add support for Fowarded header (RFC 7239) #2237 from this discussion.