Only revoke GRANT OPTION when user actually has it

When revoking privileges from a user, the GRANT OPTION is always
revoked, even if the user doesn't have it. If the user exists, this
doesn't give an error, but if the user doesn't exist, it does:

mysql> GRANT ALL ON test.* TO 'test'@'localhost';
Query OK, 0 rows affected (0.00 sec)

mysql> REVOKE GRANT OPTION ON test.* FROM 'test'@'localhost';
Query OK, 0 rows affected (0.00 sec)

mysql> REVOKE GRANT OPTION ON test.* FROM 'test'@'localhost';
Query OK, 0 rows affected (0.00 sec)

mysql> REVOKE ALL ON test.* FROM 'test'@'localhost';
Query OK, 0 rows affected (0.00 sec)

mysql> REVOKE GRANT OPTION ON test.* FROM 'test'@'localhost';
ERROR 1141 (42000): There is no such grant defined for user 'test' on
host 'localhost'

Additionally, in MySQL 5.6 this breaks replication because of
http://bugs.mysql.com/bug.php?id=68892.

Rather than revoking the GRANT OPTION and catching the error, check if
the user actually has it and only revoke it when he does.
Merge pull request #3634 from stintel/mysql-revoke-grant-option

Only revoke GRANT OPTION when user actually has it