Use default character set for expressions

- Force usage of () around complex DEFAULT expressions
- Give error if DEFAULT expression contains invalid characters
- Don't use const_charset_conversion for stored Item_func_sysconf expressions
  as the result is not constaint over different executions
- Fixed Item_func_user() to not store calculated value in str_value

sql/sql_class.h

@@ -2840,7 +2840,7 @@ public:
   bool       query_start_sec_part_used;
   /* for IS NULL => = last_insert_id() fix in remove_eq_conds() */
   bool       substitute_null_with_insert_id;
-  bool	     in_lock_tables;
+  bool	     in_lock_tables, in_stored_expression;
   bool       bootstrap, cleanup_done, free_connection_done;
 
   /**  is set if some thread specific value(s) used in a statement. */