util: Fix return state of ni_var_array_get functions

Change return code to:
 - 0  in case of object not found
 - 1  in case of object found
 - -1 in case of error

Rename ni_var_array_get_interger and ni_var_array_set_interger
to ni_var_array_get/set_uint.
client: Config prio handling based on ifindexes
Merge pull request #235 from pwieczorkiewicz/ifindextoname