Table definition cache, part 2

The table opening process now works the following way:
- Create common TABLE_SHARE object
- Read the .frm file and unpack it into the TABLE_SHARE object
- Create a TABLE object based on the information in the TABLE_SHARE
  object and open a handler to the table object

Other noteworthy changes:
- In TABLE_SHARE the most common strings are now LEX_STRING's
- Better error message when table is not found
- Variable table_cache is now renamed 'table_open_cache'
- New variable 'table_definition_cache' that is the number of table defintions that will be cached
- strxnmov() calls are now fixed to avoid overflows
- strxnmov() will now always add one end \0 to result
- engine objects are now created with a TABLE_SHARE object instead of a TABLE object.
- After creating a field object one must call field->init(table) before using it

- For a busy system this change will give you:
 - Less memory usage for table object
 - Faster opening of tables (if it's has been in use or is in table definition cache)
 - Allow you to cache many table definitions objects
 - Faster drop of table

sql/sql_class.cc

@@ -658,7 +658,8 @@ void THD::add_changed_table(TABLE *table)
 
   DBUG_ASSERT((options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)) &&
 	      table->file->has_transactions());
-  add_changed_table(table->s->table_cache_key, table->s->key_length);
+  add_changed_table(table->s->table_cache_key.str,
+                    table->s->table_cache_key.length);
   DBUG_VOID_RETURN;
 }
 
@@ -1053,7 +1054,8 @@ static File create_file(THD *thd, char *path, sql_exchange *exchange,
 
   if (!dirname_length(exchange->file_name))
   {
-    strxnmov(path, FN_REFLEN, mysql_real_data_home, thd->db ? thd->db : "", NullS);
+    strxnmov(path, FN_REFLEN-1, mysql_real_data_home, thd->db ? thd->db : "",
+             NullS);
     (void) fn_format(path, exchange->file_name, path, "", option);
   }
   else