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/sql_lex.h

@@ -592,7 +592,6 @@ typedef struct st_lex
   List<set_var_base>  var_list;
   List<Item_param>    param_list;
   SQL_LIST	      proc_list, auxilliary_table_list, save_list;
-  TYPELIB	      *interval;
   create_field	      *last_field;
   char		      *savepoint_name;		// Transaction savepoint id
   udf_func udf;