Fixed miscs problems in UMS related to users

  ** The default value of user privilige in EMF objects is not longer 0. Indeed, that was a cause of troubles to know if user set privilege when running vishnu_update_user
  ** A control is now introduced concerning the status a account when updating the user account. Previously, it was possible to delete a user like this : vishnu_update_user <user> -s 2.
     And with other status than 0, 1 or 2, it was subject to segfault.
  ** Removed the including of namespace std in the header of ums_api.cpp