improve the gotd-secrets.conf syntax don't reuse the username as the label since it makes impossible to have multiple entries with the same username. ok stsp@

allow gotd_secrets_free() to be called with NULL pointer; fixes gotd regress ok op@

initial gotd-secrets.conf implementation This moves the handling of the secrets outside gotd.conf. There will be a few changes to this, committing it as-is to continue hacking in tree. ok stsp@