+/** @brief Find existing or Create new FIB based on index
+ @param im @ref ip4_main_t
+ @param table_index_or_id - overloaded parameter referring
+ to the table or a table's index in the FIB vector
+ @param flags - used to check if table_index_or_id was a table or
+ an index (detected by @ref IP4_ROUTE_FLAG_FIB_INDEX)
+ @return either the existing or a new ip4_fib_t entry
+*/