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_select.cc

@@ -7118,6 +7118,26 @@ Item_bool_func2::add_key_fields_optimize_op(JOIN *join, KEY_FIELD **key_fields,
 }
 
 
+void
+Item_func_truth::add_key_fields(JOIN *join,
+                                KEY_FIELD **key_fields,
+                                uint *and_level,
+                                table_map usable_tables,
+                                SARGABLE_PARAM **sargables)
+{
+  if (is_local_field(args[0]))
+  {
+    Item *tmp= args[0]->type_handler()->create_boolean_false_item(join->thd);
+    if (unlikely(!tmp))
+      return;
+    add_key_equal_fields(join, key_fields, *and_level, this,
+                         (Item_field*) args[0]->real_item(),
+                         false/*equal_func*/,
+                         &tmp, 1, usable_tables, sargables);
+  }
+}
+
+
 void
 Item_func_null_predicate::add_key_fields(JOIN *join, KEY_FIELD **key_fields,
                                          uint *and_level,