r19957: Initial framework to make winbindd robust

r19957: Initial framework to make winbindd robust
against tdb corruption. Needs fleshing out
(and I forgot one record type) and needs helpful
suggestion from Volker to validate freelist,
but should give an idea of how this will look.
Jeremy.
(This used to be commit 8eb53f74e414483afde7b1e38ea2a3f56ae3ec66)