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/item_cmpfunc.cc

@@ -1281,6 +1281,37 @@ bool Item_func_truth::val_bool()
 }
 
 
+bool Item_func_truth::count_sargable_conds(void *arg)
+{
+  ((SELECT_LEX*) arg)->cond_count++;
+  return 0;
+}
+
+
+Item *Item_func_istrue::negated_item(THD *thd) const
+{
+  return new (thd->mem_root) Item_func_isnottrue(thd, args[0]);
+}
+
+
+Item *Item_func_isnottrue::negated_item(THD *thd) const
+{
+  return new (thd->mem_root) Item_func_istrue(thd, args[0]);
+}
+
+
+Item *Item_func_isfalse::negated_item(THD *thd) const
+{
+  return new (thd->mem_root) Item_func_isnotfalse(thd, args[0]);
+}
+
+
+Item *Item_func_isnotfalse::negated_item(THD *thd) const
+{
+  return new (thd->mem_root) Item_func_isfalse(thd, args[0]);
+}
+
+
 void Item_in_optimizer::fix_after_pullout(st_select_lex *new_parent,
                                           Item **ref, bool merge)
 {