recommit of Addendum to bug #29325 to 5.1.22 tree

keep_files_on_create made a startup option

sql/mysqld.cc

@@ -5081,7 +5081,8 @@ enum options_mysqld
   OPT_SECURE_FILE_PRIV,
   OPT_MIN_EXAMINED_ROW_LIMIT,
   OPT_LOG_SLOW_SLAVE_STATEMENTS,
-  OPT_OLD_MODE
+  OPT_OLD_MODE,
+  OPT_KEEP_FILES_ON_CREATE
 };
 
 
@@ -5951,6 +5952,11 @@ log and this option does nothing anymore.",
    (uchar**) &max_system_variables.join_buff_size, 0, GET_ULONG,
    REQUIRED_ARG, 128*1024L, IO_SIZE*2+MALLOC_OVERHEAD, ~0L, MALLOC_OVERHEAD,
    IO_SIZE, 0},
+  {"keep_files_on_create", OPT_KEEP_FILES_ON_CREATE,
+   "Don't overwrite stale .MYD and .MYI even if no directory is specified.",
+   (gptr*) &global_system_variables.keep_files_on_create,
+   (gptr*) &max_system_variables.keep_files_on_create,
+   0, GET_BOOL, OPT_ARG, 0, 0, 0, 0, 0, 0},
   {"key_buffer_size", OPT_KEY_BUFFER_SIZE,
    "The size of the buffer used for index blocks for MyISAM tables. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common.",
    (uchar**) &dflt_key_cache_var.param_buff_size,