New logic to check for existence of an encrypted password file.
If the encrypted file exists, skip the check for the cleartext file.
If the encrypted file does not exist, check for the cleartext file. If
found, display a warning that the encrypted file does not exist,
but indicate that the cleartext file does. This is a nudge to start
using encrypted files.
If neither files is found, display an error (unless warning only mode
was explicitly requested with '-warn passwd').