Bug #30355: Incorrect ordering of UDF results

There's currently no way of knowing the determinicity of an UDF.
And the optimizer and the sequence() UDFs were making wrong
assumptions about what the is_const member means.
Plus there was no implementation of update_system_tables()
causing the optimizer to overwrite the information returned by
the <udf>_init function.

Fixed by equating the assumptions about the semantics of 
is_const and providing a implementation of update_used_tables().
Added a TODO item for the UDF API change needed to make a better 
implementation.

include/mysql_com.h

@@ -393,6 +393,10 @@ typedef struct st_udf_init
   char	  *ptr;				/* free pointer for function data */
   my_bool const_item;			/* 0 if result is independent of arguments */
 } UDF_INIT;
+/* 
+  TODO: add a notion for determinism of the UDF. 
+  See Item_udf_func::update_used_tables ()
+*/
 
   /* Constants when using compression */
 #define NET_HEADER_SIZE 4		/* standard header size */