Handle X-Forwarded-By and X-Forwarded-Proto in make_absolute_url(). Partially fixes [bug 7221].
Dropped X-Forwarded-By for the more common X-Forwarded-Host. Updates [bug 7221].
Care about potential paths in id->url_base() as well. Updates [bug 7221].
Take special ports into consideration if X-Forwarded-Host is missing. Updates [bug 7221].