Remove (false) distinction between llog_catalog_hdr and llog_object_hdr -

Remove (false) distinction between llog_catalog_hdr and llog_object_hdr -
  they have to be the same for LOV-nested logfiles to work.
Change llog_current_log() to have an explicit pointer to the current log for
  that catalog, so we don't get confused by LOV-nested logfiles on the list.
Mark the LOV catalogs initialized so we don't create a new log each time.
Use the right message for the unlink cookies being sent to the OSTs.
Set the MDS unlink record type.