(services.inc / services_radvd_configure) more duplication for https://github.com/opnsense/core/issues/1196

(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)

(services) add support for "non stoppable" services, configuration options without status. needed for https://github.com/opnsense/core/issues/1217

Spotted by: pavelb

Add translatable template

While there, kill "if (write_config())" constructs as this
really isn't a thing.

Not sure if this works, but it looks better than before.

This doesn't do anything in particular, because we don't know
what the input is without parsing it further.  It's not worse
than before in any case.  ;)

We don't need 7z anymore, so it's better to use the lightweight package.

(ipsec, "save password") change yes -> 1 for 28673. for https://github.com/opnsense/core/issues/1209

It's only partially true and will be compeltely wrong in
the mid-term future.  ;)