network: several cleanups for link_reconfigure()

network: several cleanups for link_reconfigure()

Effectively no functional changes, just refactoring and preparation for
later changes.

- convert boolean flag 'force' to LinkReconfigurationFlag enum,
- merge link_reconfigure() and reconfigure_handler_on_bus_method_reload() as
  link_reconfigure_full(),
- Rename ReconfigureData -> LinkReconfigurationData,
- make Reconfigure() DBus message wait for reconfiguration being
  started before sending reply.