Add a new account expiration feature to the password policy.  This will make it
possible for accounts to be given an expiration time, after which it will not
be possible to authenticate as that user or target that user with the proxied
authorization control.

If the ds-pwp-account-expiration-time operational attribute is included in the
user's entry and it references a time in the past, then the user's account will
be considered expired.  If it references a time in the future, then it will be
expired at that time.  If this attribute is absent, then the account will not
expire.

OpenDS Issue Number:  543