Support NVARCHAR type.

[winkyao]

MySQL will convert NVARCHAR to VARCHAR, but TiDB will return error.

MySQL:

mysql> create table t1(a NVARCHAR(100));
Query OK, 0 rows affected (0.01 sec)

mysql> desc t1;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| a     | varchar(100) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> drop table t1;
Query OK, 0 rows affected (0.21 sec)

mysql> create table t1(a NVARCHAR(100));
ERROR 1105 (HY000): line 0 column 26 near "(100))" (total length 32)