(services.inc / services_radvd_configure) fix for https://github.com/opnsense/core/issues/1196 and cleanups

Quite some inconsistencies in this function, parameters that can't be set, lack of logic and code duplication.
* removes get_configured_pppoe_server_interfaces(), which can't work anyway... pppoe server interfaces are registered via the plugin itself.
* $dhcpv6ifconf['mode'] can't be set anywhere....
* don't try to revalidate rainterface
* collect "real interface" from dhcpif, shouldn't use carp if
* remove code duplication in dns collection
* move checks up for interface we should skip (when possible)