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.

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.