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.

This has the advantage the transactions will also use it, preventing them
from blocking.

This reduces the number of huge allocs, which drops test time from 6 minutes to 57 seconds.

Inefficient scheme for allocations > 1/8192 of the pool size.

We can have a 0 byte in the *middle* of an encoding.

We can't allow NULL with the new variant (needed by talloc's set_destructor
for example), so document that and expose all three variants for different
uses.

cast_if_type() should not try to degrade the expression using 1?(test):0,
as that promotes bool to int, as well as degrading functions to function
pointers: it should be done by the callers.

In particular, this fixes sparse_bsearch.

This means we can't have more than 2^25 elements per page; that's
a maximum small page size of about 2^24 (with >8 objects per small page
we move to large pages), meaning a poolsize max of 4G.

We have a tighter limit at the moment anyway, but we should remove it
once we fix this.  In particular count all-zero and all-one words in
the used field (that's what we care about: full or empty) would give us
another factor of 64 (we only care about larger pool sizes on 64-bit
platforms).

We can also restore the larger number of pages and greater inter-page
spacing once we implement the alternative tiny allocator.

Turns out that now we use page numbers, this is more fundamental.

This version has limitations: pools must be at least 1MB, and allocations
are restricted to 1/1024 of the total pool size.