Update the modify operation to fix two problems:

- When the "force change on add" or "force change on reset" was enabled in the
  password policy and the user's password had been administratively reset, it
  was still possible for that user to perform modify operations against entries
  other than their own.  Issue #904.
  
- When the "force change on add" feature was enabled but "force change on 
  reset" was not, the server would not clear the reset flag when a user changed
  his or her password via a modify operation.  Issue #905.