Make a change to the account status notification handler API so that it is
possible to provide additional properties along with the notification.  This
makes it possible to develop account status notification handlers that can act
more intelligently and/or provide more useful information.

This also reverts an earlier change that migrated hard-coded strings to
localizeable messages for notification types.  The set of notification types
should in fact be hard-coded strings because they may be used in the
configuration, and the contents of a configuration file should not depend on
the locale in which the server is operating.