Give a better error message when userdb-only checkpassword program is used with passdb checkpassword.
Give a better error message when userdb-only checkpassword program is used with passdb checkpassword.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot