Fixed BUG#4902: Stored procedure with SHOW WARNINGS leads to packet error,

and tried to do it properly this time, digging out all show commands that
  need the multi results flag set.

sql/sp_head.h

@@ -32,6 +32,9 @@
 Item_result
 sp_map_result_type(enum enum_field_types type);
 
+bool
+sp_multi_results_command(enum enum_sql_command cmd);
+
 struct sp_label;
 class sp_instr;
 struct sp_cond_type;