Field type year should have unsigned flag

[jackysp]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

drop table if exists t;
create table t (
	c_year year
);

insert into t values (
	11
);

select * from t;

2. What did you expect to see?

Field   1:  `c_year`
Catalog:    `def`
Database:   `mysql`
Table:      `t`
Org_table:  `t`
Type:       YEAR
Collation:  binary (63)
Length:     4
Max_length: 4
Decimals:   0
Flags:      UNSIGNED ZEROFILL NUM 


+--------+
| c_year |
+--------+
|   2011 |
+--------+
1 row in set (0.00 sec)

3. What did you see instead?

Field   1:  `c_year`
Catalog:    `def`
Database:   `test`
Table:      `t`
Org_table:  `t`
Type:       YEAR
Collation:  binary (63)
Length:     16
Max_length: 4
Decimals:   0
Flags:      BINARY NUM 


+--------+
| c_year |
+--------+
|   2011 |
+--------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V)?