Patch for WL#3736: Extended Table, Column and Index Comments.

The task is to 
  (a) add a comment on indexes and 
  (b) increase the maximum length of column, table and the new index comments.

The patch committed on behalf of Yoshinori Matsunobu (Yoshinori.Matsunobu@Sun.COM).

sql/structs.h

@@ -102,6 +102,7 @@ typedef struct st_key {
     int  bdb_return_if_eq;
   } handler;
   TABLE *table;
+  LEX_STRING comment;
 } KEY;