tdb: put example hashes into header, so we notice incorrect hash_fn.

tdb: put example hashes into header, so we notice incorrect hash_fn.

This is Stefan Metzmacher <metze@samba.org>'s patch with minor changes:
1) Use the TDB_MAGIC constant so both hashes aren't of strings.
2) Check the hash in tdb_check (paranoia, really).
3) Additional check in the (unlikely!) case where both examples hash to 0.
4) Cosmetic changes to var names and complaint message.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(cherry picked (modified for v3-4) from commit 786b7263000dedcb97e7369402e2e9dc967e36c4)

Signed-off-by: Stefan Metzmacher <metze@samba.org>