Bug #6379: ENUM values are incorrectly converted

- add_field_to_list() now uses <List>String
instead of TYPELIB to be able to distinguish
literals 'aaa' and hex literals 0xaabbcc.
- move some code from add_field_to_list() where
  we don't know column charset yet, to 
  mysql_prepare_table(), where we do.

sql/field.h

@@ -1198,6 +1198,7 @@ public:
   uint decimals,flags,pack_length;
   Field::utype unireg_check;
   TYPELIB *interval;			// Which interval to use
+  List<String> interval_list;
   CHARSET_INFO *charset;
   Field::geometry_type geom_type;
   Field *field;				// For alter table