r23930: Introduce tdb_validate_and_backup:

r23930: Introduce tdb_validate_and_backup:
This is a wrapper around tdb_validate, that does backup handling:

* if the given tdb is valid, a backup is created (name.bak)
* if the tdb is invalid, if a valid bakup is found (validated
  with the same validation function) under the name "name.bak"
  the orignal tdb is moved to name.corrupt and the backup
  is restored.

For the backup handling, a variant of the backup_tdb function
from lib/tdb/tools/tdbbackup.c is included in lib/util_tdb.c.
The copy function for the traverse action eliminates the need
to maintain a global success state by using a struct wrapping
the target tdb and a success flag as the private data.

Michael
(This used to be commit ca63a99b6188c0976603dc2852e478cc7a6b2f8b)