* Produce right results for conditions that were transformed to "(partitioning_range) AND
  (list_of_subpartitioning_ranges)": make each partition id set iterator auto-reset itself
  after it has returned all partition ids in the sequence 
* Fix "Range mapping" and "Range mapping" partitioning interval analysis functions to 
  correctly deal with NULL values. 


mysql-test/r/partition_pruning.result:
  Testcase for BUG#18558
mysql-test/t/partition_pruning.test:
  Testcase for BUG#18558
sql/opt_range.cc:
  BUG#18558: Move partition set iterator initialization to sql_partition.cc, comment fixes
sql/partition_info.h:
  BUG#18558: Make each partition set iterator auto-reset itself after it has returned all 
  partition ids in the set it enumerates.
sql/sql_partition.cc:
  BUG#18558: 
  - Make each partition set iterator auto-reset itself after it has returned all 
    partition ids in the set it enumerates. 
  - Fix partition interval analysis to correctly handle intervals with one or both
    NULL bounds.
sql/sql_partition.h:
  BUG#18558: 
   - Make each partition set iterator auto-reset itself after it has returned all 
     partition ids in the set it enumerates. 
   - Rename PARTITION_ITERATOR::has_null_value to ret_null_part