Made it possible to use --max-delayed-threads=0 in order to disable

INSERT DELAYED.

sql/mysqld.cc

@@ -3770,9 +3770,9 @@ struct my_option my_long_options[] =
    (gptr*) &max_connect_errors, (gptr*) &max_connect_errors, 0, GET_ULONG,
     REQUIRED_ARG, MAX_CONNECT_ERRORS, 1, ~0L, 0, 1, 0},
   {"max_delayed_threads", OPT_MAX_DELAYED_THREADS,
-   "Don't start more than this number of threads to handle INSERT DELAYED statements.",
+   "Don't start more than this number of threads to handle INSERT DELAYED statements. This option does not yet have effect (on TODO), unless it is set to zero, which means INSERT DELAYED is not used.",
    (gptr*) &max_insert_delayed_threads, (gptr*) &max_insert_delayed_threads,
-   0, GET_ULONG, REQUIRED_ARG, 20, 1, 16384, 0, 1, 0},
+   0, GET_ULONG, REQUIRED_ARG, 20, 0, 16384, 0, 1, 0},
   {"max_heap_table_size", OPT_MAX_HEP_TABLE_SIZE,
    "Don't allow creation of heap tables bigger than this.",
    (gptr*) &global_system_variables.max_heap_table_size,

sql/sql_insert.cc

@@ -140,7 +140,8 @@ int mysql_insert(THD *thd,TABLE_LIST *table_list, List<Item> &fields,
   if ((lock_type == TL_WRITE_DELAYED &&
        ((specialflag & (SPECIAL_NO_NEW_FUNC | SPECIAL_SAFE_MODE)) ||
 	thd->slave_thread)) ||
-      (lock_type == TL_WRITE_CONCURRENT_INSERT && duplic == DUP_REPLACE))
+      (lock_type == TL_WRITE_CONCURRENT_INSERT && duplic == DUP_REPLACE) ||
+      !max_insert_delayed_threads)
     lock_type=TL_WRITE;
 
   if (lock_type == TL_WRITE_DELAYED)