services: cleanups
services: cleanups

* In override mode turn radvd off until enabled, same as DHCPv6
* Guard radvd against is_radvd_enabled(), same as DHCPv6
* Avoid extra hoops of using global $config where possible

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core