This means that older code will not be able to open new TDBs with
a non-default hash, *even* if they are using the correct hash.
Non-default hashes were unusual, but now SAMBA is considering using
a non-default hash, and avoiding corruption seems more important
than backwards compatibility for an obscure case.

We currently allow opening of a TDB without any idea if we're using the
right hash function.  Since most people use the default, this hasn't been
a big issue.

The next change will put an example hash value in the rwlocks field in
the header: current TDB already refuses to open a TDB with this field
non-zero.  This commit simply adds tests that that occurs.

We changed other tests to run in our tmpdir, missed this one.

I also changed -x to be specified multiple times rather than using , separators.

As the locking issues with enlarging a hash were so nasty, we switch to a
tree structure for the entries.  It's a hash which expands to point to
sub-hashes when it fills.

This means we no longer have a 'volatile' header: the top hash cannot move.
In fact, we no longer store a copy of the header in the tdb_context; we only
need hash_seed.

New helper functions for accessing writable areas and committing the results
(if they had to be copied).  New debug test to make sure we don't hold access
while we're doing something which could cause us to unmap/remap.

Find becomes more complicated: we need to track where we found (or didn't
find) an entry so we can easily add/delete it.

Traverse becomes more complicated: we need to track where we were in the
hash tree.

This eliminates all the "drop lock and retry" logic: we expand inside alloc().

We put the free lists at the beginning of a zone; this means no record
can be larger than a zone, but means they cannot move.  Once we change
hashes to be expanding, they won't move either and the result should be
simpler.

The unreserved headers were a real bug; the others could cause a false error too.

We were not actually recording valgrind failures in ccanlint!

These tests revealed more bugs with delete (rewritten) and hash enlargement.

Use hash_add() when resizing hash, too.

This explains why most of tdb_gradual_lock was not tested: thanks gcov!

eg. -v dumps percentage of coverage, -vv dumps per-line data, -vvv dumps every command executed.

This means parallel "make check" works again.