Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions

- Honor unsigned_flag in the corresponding functions
- Use compare_int_signed_unsigned()/compare_int_unsigned_signed() instead of explicit comparison in GREATEST() and LEAST()

sql/sql_class.h

@@ -1460,6 +1460,7 @@ class user_var_entry
   char *value;
   ulong length, update_query_id, used_query_id;
   Item_result type;
+  bool unsigned_flag;
 
   double val(my_bool *null_value);
   longlong val_int(my_bool *null_value);