[bug-mailutils] POP3d enhancements

[Alain Magloire]

Like to propose a few enhancement of the next version of POP3D

LOGIN-DELAY:
        We could do this by creating a pipe(2) between the child and the parent
        - when the child is  in UPDATE state (after the QUIT)
          it write(2) to pipe the username
        - the parent read(2) the pipe(2) and save the info in a list with a 
timestamp, time(2)
        - On new connection, after the fork(2), in the child, the timestamp is 
check
          error code "-ERR [LOGIN-DELAY]" if delay not expired. 
        - option --login-delay=seconds (default to NEVER)

EXPIRE:
        Implemented with a "X-Expire" header field, when a message been acted 
upon
        via RETR, the header is added to the message, the value is a timestamp, 
time(2).
        On UPDATE state expired messages are deleted.
        - option --expire=days (default to NEVER)

RFC-3206:
        SYS/AUTH, new POP3 rfc out.