- drop "goto error_label" in favor of returning right away.
  At most places, there was no need to do any cleanup or
  the cleanup is handled via nm_auto().

- adjust the return types of wireguard functions to return
  a boolean success/failure, instead of some error code which
  we didn't use.

- the change to _wireguard_get_link_properties() is intentional.
  This was wrong previously, because in _wireguard_get_link_properties()
  obj is always a newly created instance, and never has a genl
  family ID set. This will be improved later.