networkctl: open the bus just once

networkctl: open the bus just once

We'd connect to the bus twice: the first time to check networkd namespace,
and then the second time to do the deed we were asked to do. It's nicer
to open the bus just once, for efficience and also to avoid the open call
in all functions.

An ASSERT_PTR helper is added:
- sd_bus *bus = userdata;
  ...
- assert(bus);
+ sd_bus *bus = ASSERT_PTR(userdata);
  ...

It can be used in other place too, but I'm leaving that for a later
refactoring.