Measured against a running server built from this module's own rendered
config, not a hand-written one -- which is the fix to the method as much
as to the file. Two shapes died in a row: a bare name authenticates
anyone who sends it, and a name plus an nkey refuses to start at all
(nkey users do not take usernames or passwords).
A malformed key is fail-closed too, so the only way to a live server
here is a real key whose seed nobody but the responder holds.
a8c5877412