r11761: * fix clearing of event logs by truncating the tdb.

r11761: * fix clearing of event logs by truncating the tdb.
  This feature got broken in some of the other updates.
  Now each open handle stores an pointer to an open tdb
  data structure (not the tdb pointer itself).
  Clearing can be done with a simple elog_close_tdb( elog, True )
  to force a close and then calling elog_open_tdb( logname, True )
  to force an tdb truncate.  Permissions on existing tdbs are
  maintained which is important.

* We don't currently handle backup.  Haven't looked at the
  format of a backuped up eventlog to know what the deal is.
(This used to be commit 2df34c9403446d12f1ceeac38cbda5d3ba805b02)