Interface differences between TDB and NTDB.

- ntdb shares 'struct TDB_DATA' with tdb, but TDB defines the TDB_DATA
  typedef, whereas ntdb defines NTDB_DATA (ie. both are compatible).
  If you include both ntdb.h and tdb.h, #include tdb.h first,
  otherwise you'll get a compile error when tdb.h re-defined struct
  TDB_DATA.

- ntdb functions return NTDB_SUCCESS (ie 0) on success, and a negative
  error on failure, whereas tdb functions returned 0 on success, and
  -1 on failure.  tdb then used tdb_error() to determine the error;
  this API is nasty if we ever want to support threads, so is not supported.

- ntdb's ntdb_fetch() returns an error, tdb's returned the data directly
  (or tdb_null, and you were supposed to check tdb_error() to find out why).

- ntdb's ntdb_nextkey() frees the old key's dptr, in tdb you needed to do
  this manually.

- tdb's tdb_open/tdb_open_ex took an explicit hash size.  ntdb's hash table
  resizes as required.

- ntdb uses a linked list of attribute structures to implement logging and
  alternate hashes.  tdb used tdb_open_ex, which was not extensible.

- ntdb does locking on read-only databases (ie. O_RDONLY passed to ntdb_open).
  tdb did not: use the NTDB_NOLOCK flag if you want to suppress locking.

- ntdb's log function is simpler than tdb's log function.  The string is
  already formatted, and it takes an enum ntdb_log_level not a tdb_debug_level,
  and which has only three values: NTDB_LOG_ERROR, NTDB_LOG_USE_ERROR and
  NTDB_LOG_WARNING.

- ntdb provides ntdb_deq() for comparing two NTDB_DATA, and ntdb_mkdata() for
  creating an NTDB_DATA.

- ntdb's ntdb_name() returns a copy of the name even for NTDB_INTERNAL dbs.

- ntdb does not need tdb_reopen() or tdb_reopen_all().  If you call
  fork() after during certain operations the child should close the
  tdb, or complete the operations before continuing to use the tdb:

	ntdb_transaction_start(): child must ntdb_transaction_cancel()
	ntdb_lockall(): child must call ntdb_unlockall()
	ntdb_lockall_read(): child must call ntdb_unlockall_read()
	ntdb_chainlock(): child must call ntdb_chainunlock()
	ntdb_parse() callback: child must return from ntdb_parse()

- ntdb will not open a non-tdb file, even if O_CREAT is specified.

- There is no ntdb_traverse_read.  For operating on TDB files, you can
  simulate it by ntdb_add_flag(tdb, NTDB_RDONLY); ntdb_traverse();
  ntdb_remove_flag(tdb, NTDB_RDONLY).  This may be desirable because
  traverse on TDB files use a write lock on the entire database
  unless it's read-only.

- Failure inside a transaction (such as a lock function failing) does
  not implicitly cancel the transaction; you still need to call
  ntdb_transaction_cancel().

- There is no NTDB_CLEAR_IF_FIRST flag; it has severe scalability and
  API problems.  If necessary, you can emulate this by using the open
  hook and placing a 1-byte lock at offset 4.  If your program forks,
  you will need to place this lock again in the child.