OPENDJ-2294 ldapmodify ds-pwp-account-disabled does not generate account status notifications

Some code was lost while implementing pass through authentication, a.k.a. PTA (OPENDJ-262), at SVN revision 7203.
However, it looks like this code was already dead at that moment.

LocalBackendModifyOperation.java:
In handleInitialPasswordPolicyProcessing(), added code that checks whether the account is enabled or not.
The code generating the account status notification already exists.
Extracted method isInternalOrSynchro().