We don't use it for the moment, but Brad Hards noted that assigning to
dest ten continuing was pointless, revealing this bug.

We only ever write to this variable, so can just be removed.

This is always overwritten later in the function.

obe is always overwritten later.

last is unconditionally set to zero later in this function.

lp_bits was only ever written to.

asprintf is a PITA to use, and it's not in POSIX anyway.  Provide
replacements, and also provide a nicer-to-use afmt() wrapper.

tools/ccan_depends --compile ccan/module (without a trailing slash)
results in a segfault.

(Patched forward by Rusty)

We get a point for every run or api test, rather than 1 point for all of them,
so be consistent.

cast package has all negative tests, plus testable examples.

Thanks to Andreas Schlick, we have a nicer test for when gcc warns about
trampolines (gcc 4.6's -Wtrampolines).  This works at any optimization level,
and means when that warning is enabled we recognize that we shouldn't allow
nested functions.

Increase from:
	1985 of 2553 lines covered
to:
	2020 of 2552 lines covered

Increase from:
	1985 of 2553 lines covered
to:
	2018 of 2552 lines covered

Increase from:
	1962 of 2553 lines covered
to:
	1985 of 2553 lines covered

The "ret < n" was done as an unsigned comparison, so it didn't work as
expected when ret was negative.

Simplest fix is to do an equals comparison everywhere, which is also
slightly stricter.

You can always specify the TDB_NOLOCK flag along with O_RDONLY for the old
behaviour.

If benchmarking indicates a problem later, we can restore it.

Mostly a fairly simple transformation, since 0 still means success.

One new twist is that tdb_nextkey now frees the .dptr of the key; this
us usually what we want but does cause issues for our weird test code.

Of course, we leave the API the same, but percolate error codes all the
way back to tdb_check().

In particular, tdb_needs_recovery() can now indicate an error occurred,
rather than returning true.

This time we have to use our tri-value "tdb_bool_err" type to indicate
true, false, or error, which now allows us to correctly handle errors
in key matching (rather than treating it as a non-match).

In particular, we replace the TDB_OFF_ERR ((off_t)-1) with a range of
negative error values.

This causes a subtle enhancement in tdb_direct(): it previously
returned NULL on both "can't use direct access" or "some error
occurred", as the caller always uses read/write functions as a
fallback anyway.  Now we distinguish the error case.

We have a series of I/O functions which change depending on whether we're
inside a transaction or not.  This makes them return enum TDB_ERROR instead
of int.

This snuck in fe55330a which added the stats attribute.  Without it,
TDB works but is vulnerable to segmenation faults or write errors when
disk is exhausted.

Make the other functions in lock.s return an error code, too.

We make tdb_brlock() and tdb_lock_gradual() return an error code directly:
the callers have to set tdb->ecode. 

I use the "high pointers hold error numbers" trick, and also make
tdb_logerr return the error code, which enables the common case of
"return tdb_logerr(...)".