initdb check_need_password fix

If you run initdb -A md5, you get an error

initdb: must specify a password for the superuser to enable md5 authentication

In 9.2, when you run something like initdb --auth-local=peer
--auth-host=md5, you still get that error, which I think is a mistake.
The error should only be shown if both authentication choices are
password-like.  Hence I propose the attached patch.