[ticket/10327] Change CREATE INDEX to ALTER TABLE table ADD INDEX for MySQL.

* CREATE INDEX is internally mapped to an ALTER INDEX statement.
* CREATE INDEX requires the INDEX permission.
* ALTER TABLE requires the (more powerful) ALTER permission.
* We require the ALTER permission anyway for operation.
* Changing CREATE INDEX to ALTER TABLE thus removes dependency on the INDEX
  permission which is good because some management software does not give
  out the INDEX permission by default.

http://dev.mysql.com/doc/refman/5.0/en/create-index.html

PHPBB3-10327

phpBB/includes/db/db_tools.php

@@ -2145,7 +2145,7 @@ class phpbb_db_tools
 				}
 			// no break
 			case 'mysql_41':
-				$statements[] = 'CREATE INDEX ' . $index_name . ' ON ' . $table_name . '(' . implode(', ', $column) . ')';
+				$statements[] = 'ALTER TABLE ' . $table_name . ' ADD INDEX ' . $index_name . '(' . implode(', ', $column) . ')';
 			break;
 
 			case 'mssql':