gh-664: role accepts bad syntax for create

A fix and a test case for gh-664 (bad syntax is accepted in
schema.role.*, schema.user.*, etc).

Check the second argument of role.create()/role.drop(),
user.create()/user.drop(), func.create()/func.drop().

Implement if_exists option for user.drop(), role.drop(),
func.drop().

Add a test case.