Log Message:
Add checkpw: Password-checking tools for password files in Maildir

checkpw is a simple and flexible checkpassword interface package.

checkpw checks password which is stored in a file in the Maildir
directory. The file has read and write permissions to the user only.
checkpw checks real user's password.

- checkpw allows POP authentication.
- checkapoppw allows APOP authentication.
- Multiple maildirs per POP3 user are supported.
- Multiple password per maildir are supported.

Before invoking following program, checkpw and checkapoppw set up $USER,
$EXT, $HOME, $SHELL, its supplementary groups, its gid, its uid, and its
working directory.