This makes transition from tdb1 much simpler.

As per TDB1, except that the sequence number is 63 bit.

As per TDB1, with one enhancement: a non-NULL name argument passed to
tdb_open() with the TDB_INTERNAL flag is preserved so you can identify
internal TDBs too.

We don't need this now, since we use explicit error numbers.

Everyone writes their own, so provide a standard one.

These are the same as the TDB1 functions; but note that there is no reliable
way to tell if tdb_exists() fails due to an error.  This simplifies the API,
but means you have to use tdb_fetch() if you really care. 

Finally, we enable sharing of the struct tdb_file.  We add a reference count
so we can free the struct tdb_file when the last tdb is closed.

The last step before sharing struct tdb_file is to add a field to
identify which tdb context is holding each lock, so we can fail when
they conflict.

Internal reshuffle which makes things shorter and neater.

It makes sense to share the mmap between multiple openers.

This moves the fd and locking information into a new 'struct tdb_file',
opening the way for it to be shared by multiple tdb_open calls on the
same file.

Simply tidying up.

As detailed in doc/design.lyx section 2.16 "Record Headers Are Not
Expandible", we make sure that if there is padding at the end of a record,
the first byte of padding is a zero.

As detailed in doc/design.lyx section 2.15 "Extending The Header Is
Difficult"; we add features_used and features_offered fields to the
header, so we can identify if we add new features, and then if someone
opens it who doesn't understand that feature.

This make sure that the data being handed to the function in tdb_traverse
is the correct type.

This is vitally important for forwards compatibility.

Two missed cases in b2100462 (tdb2: change API to return the error
value.)

It's now important, since it defines _GNU_SOURCE.

Recent changes shifted line numbers in tap.c, so the break is now in
the wrong place.  We should probably have an explicit function we can
breakpoint instead.

Enable CCAN_STR_DEBUG in the default flags, so our tools get checked,
and fix up the resulting errors.

The standard ctype.h variants will make you go insane if you
contemplate them for too long.

isspace() and isdigit() want an unsigned value.

tolower(), toupper() and isupper want an unsigned value.  Also, fix
lazy cost-discarding inside strrchr for extra safety.

If we detect any mention of a problematic string function, try compiling
the entire module with string debugging enabled.

In the former case, we were bitten by the fact that you don't pass a char
to isalpha() et al: you pass an int.  This means on most platforms you want
to do:

   if (isalpha((unsigned char)c)) ...

Insane?  Yes, but I assure you I'm not making this up.

Similarly, I've always wanted strstr, strchr and strrchr to return
const char * when given a const char * argument to search, to avoid
constness leak.

In both cases, the actual versions from the headers may be macros, so
we need to be very careful overriding them.  The result is that they
become out-of-line functions which is unacceptable for general
performance.

So we only activate these when CCAN_STR_DEBUG is defined.

Same thing (a BUILD_ASSERT which evaluates to zero), but there's a
strong preference for all modules to stick with their own names as
prefixes.

They're often out-of-date, and we're about to do special things to the
str module compile flags, so we don't *ever* want the default.

It doesn't add much to build times, and it eliminates a nasty source
of weird errors.

We need to flush stdout before forking.

Also add a note about a possible segfault.

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