MDEV-34189 Unexpected error on WHERE inet6col

normalize_cond() translated `WHERE col` into `WHERE col<>0`

But the opetator "not equal to 0" does not necessarily exists
for all data types.

For example, the query:

  SELECT * FROM t1 WHERE inet6col;

was translated to:

  SELECT * FROM t1 WHERE inet6col<>0;

which further failed with this error:

  ERROR : Illegal parameter data types inet6 and bigint for operation '<>'

This patch changes the translation from `col<>0` to `col IS TRUE`.
So now
  SELECT * FROM t1 WHERE inet6col;
gets translated to:
  SELECT * FROM t1 WHERE inet6col IS TRUE;

Details:
1. Implementing methods:
   - Field_longstr::val_bool()
   - Field_string::val_bool()
   - Item::val_int_from_val_str()
   If the input contains bad data,
   these methods raise a better error message:
     Truncated incorrect BOOLEAN value
   Before the change, the error was:
     Truncated incorrect DOUBLE value

2. Fixing normalize_cond() to generate Item_func_istrue/Item_func_isfalse
   instances instead of Item_func_ne/Item_func_eq

3. Making Item_func_truth sargable, so it uses the range optimizer.
   Implementing the following methods:
   - get_mm_tree(), get_mm_leaf(), add_key_fields() in Item_func_truth.
   - get_func_mm_tree(), for all Item_func_truth descendants.

4. Implementing the method negated_item() for all Item_func_truth
   descendants, so the negated item has a chance to be sargable:
   For example,
     WHERE NOT col IS NOT FALSE    -- this notation is not sargable
   is now translated to:
     WHERE col IS FALSE            -- this notation is sargable

sql/sql_type.cc

@@ -5258,7 +5258,7 @@ bool Type_handler_temporal_result::Item_val_bool(Item *item) const
 
 bool Type_handler_string_result::Item_val_bool(Item *item) const
 {
-  return item->val_real() != 0.0;
+  return item->val_bool_from_str();
 }
 
 
@@ -8162,6 +8162,12 @@ Item *Type_handler_interval_DDhhmmssff::
                                                                attr.decimals());
 }
 
+/***************************************************************************/
+Item_literal *Type_handler::create_boolean_false_item(THD *thd) const
+{
+  return new (thd->mem_root) Item_int(thd, 0);
+}
+
 /***************************************************************************/
 
 void Type_handler_string_result::Item_param_setup_conversion(THD *thd,