Log Message:
Add qmail-dovecot-checkpassword: DJB-style checkpassword for Dovecot auth socket

qmail checkpassword helper for dovecot auth socket. This can be used as
checkpassword tool from within qmail. Very useful if you use dovecot
with several authdbs and you want to ask all at once. Checks user auth
on (hardcoded) dovecot auth socket.