Make direct LDAP operation optional and add a hook for addional actions.

As the old LDAPcontrol module was configurable via hooks to manipulate the data structure of LDAP entries it should be possible to add additional actions triggered by user_update and user_delete hook usage of this module. And even if a module is using only the password synchronization it makes sense that there is as possibility to deactivate the direct LDAP operations of this module.