checks password which is stored in ~/Maildir/.password

 checkpw is an implementation of the checkpassword interface that checks a
 password against a ``.password'' file in the user's Maildir. The password
 file has read and write permissions to the user only.
 .
 The code is in the public domain.
 .
 The original checkpassword program and interface specification is written
 by D. J. Bernstein:
 http://cr.yp.to/checkpwd.html