udf_init() can crash when the name field is empty.

We correct this by applying CHECK column constraints on
columns in the mysql.func tables.

The constrains are as follows:
* name cannot be an empty string;
* ret, the return type can be;
  * STRING_RESULT(0),
  * REAL_RESULT(1),
  * INT_RESULT(2),
  * and DECIMAL_RESULT(4);
  * but not ROW_RESULT(3) or TIME_RESULT(5).
*  dl, cannot be empty string

Upgrades ensure that any invalid functions are deleted.

closes #1643