WL#1266 "Separate auto-set logic from TIMESTAMP type."

Final version of patch.

Adds support for specifying of DEFAULT NOW() and/or ON UPDATE NOW()
clauses for TIMESTAMP field definition.
Current implementation allows only one such field per table and
uses several unireg types for storing info about this properties of
field. It should be replaced with better implementation when new
.frm format is introduced.

sql/mysql_priv.h

@@ -633,7 +633,7 @@ void set_item_name(Item *item,char *pos,uint length);
 bool add_field_to_list(THD *thd, char *field_name, enum enum_field_types type,
 		       char *length, char *decimal,
 		       uint type_modifier,
-		       Item *default_value,
+		       Item *default_value, Item *on_update_value,
 		       LEX_STRING *comment,
 		       char *change, TYPELIB *interval,CHARSET_INFO *cs,
 		       uint uint_geom_type);