unit: properly update references to units which are merged
When we merge units that some kind of object points to, those pointers might become invalidated, and needs to be updated. Introduce a UnitRef struct which links up all the unit references, to ensure corrected references. At the same time, drop configured_sockets in the Service object, and replace it by proper UNIT_TRIGGERS resp. UNIT_TRIGGERED_BY dependencies, which allow us to simplify a lot of code.