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.

This prevents us from writing out gcov files.

- Neaten I/O function
- Don't use fill in zero_out: it's only for low-level ops.
- Don't mangle arg in tdb_write_convert: it broke write_header.
- More use of tdb_access_read, make it optionally converting.
- Rename unlock_range to unlock_lists.
- Lots of fixes to enlarge_hash now it's being tested.
- More expansion cases tested.