Fix that we can read tables with the 'older' decimal format used in 5.0.3 & 5.0.4

We will however give a warning when opening such a table that users should use ALTER TABLE ... FORCE to fix
the table. In future release we will fix that REPAIR TABLE will be able to handle this case

sql/sql_lex.h

@@ -635,6 +635,7 @@ typedef class st_select_lex SELECT_LEX;
 #define ALTER_CHANGE_COLUMN_DEFAULT 256
 #define ALTER_KEYS_ONOFF        512
 #define ALTER_CONVERT          1024
+#define ALTER_FORCE		2048
 
 typedef struct st_alter_info
 {