fsm: tune ni_ifworker_by_ifindex

- rename to ni_fsm_ifworker_by_ifindex
- add param validity check
fsm: Add object_path parsing functions

- __ni_fsm_dbus_objectpath_to_ifindex
- __ni_fsm_dbus_objectpath_to_name
fsm: tune ni_fsm_ifworker_by_object_path

This function looks also for not refreshed
workers by their name.
Merge pull request #301 from pwieczorkiewicz/fsm_tweaks