Item_sum::set_aggregator() may be called multiple times during query preparation.
On subsequent calls: verify that the aggregator type is the same,
and re-use the existing Aggregator.


sql/item_sum.cc:
  In Item_sum::set_aggregator(): re-use existing Aggregator if already set.
  
  Remove some friend declarations, add some accessor functions.
  Cleanup some DBUG_ENTER and DBUG_RETURN code.
sql/item_sum.h:
  Make some member fields private, add accessors instead.
  Remove some un-necessary friend declarations.
  Remove some default arguments from constructors.
sql/opt_sum.cc:
  Use accessor functions in Item_sum.
sql/sql_select.cc:
  Fix mis-spelled DBUG_ENTER text.
  Use accessor functions in Item_sum.
sql/sql_yacc.yy:
  Use explicit true/false rather than default arguments when constructing
  Item_sum_xxx objects.