Recent real usage case showed this leak when we call opt_usage_and_exit:
we don't bother freeing before exit.  With valgrind, it matters.

See https://github.com/rustyrussell/ccan/pull/1#issuecomment-2856561
for details.
(Reworked with minor cleanups -- Rusty)

Based on patch by Andrey Smirnov <andrew.smirnov@gmail.com>:
    
    There is are certain use-cases when it is necessary to know the offset
    of the member in a structure's memory layout. One such use-case can be
    seen in `ccan/list/list.h' in macros `list_for_each' and
    `list_for_each_safe'. This commit implements said functionality with
    `container_of_var_off' macro.

Renamed container_of_var_off -> container_off_var now we have container_off.

It's convenient to check that the member is the given type, but we can leave
that to the callers.

Just adding `-g' to list of CFLAGS doesn't make gcc to generate debug
information required for macro expansion during debugging. Replacing
it with `-g3 -ggdb' rectifies this.

(configurator.c commit folded by Rusty)

Nice for distributed option declaration.

Also add #include to AUTODATA_TYPE example, so ccanlint doesn't try to
put it inside a function (which works, for this, but gives a warning
about unused variables).

Makes things a bit neater.

eg. "See also" can match "see-also".

Gets rid of generated file.

This is a more portable variant of the ELF section trick.

Very slow, but sometimes you need to know without crashing.

It's much easier to simply say "keep all", and it simplifies the code
quite a bit.

I use --target far more than --timeout.

Rather than using a separate index of tetst to skip, make sure tests
we don't want to run are removed from the graph of tests to start
with.

This means that the "skip" field is only set when we fail a test (and
thus must be reset when we test the next module).

Rather than walking the list of tests manually, use
dgraph_traverse_to() to run the tests.

Instead of a linked list of tests, we use dgraph and strmap.

Make info_exists() call add_info_options() directly, rather than detecting
that we've found _info inside the loop of tests.

It means targeting is now a global, which is a bit sucky.

This way, we can debug really early failures (eg. during option
parsing).

It's reduced to a flag which means we stop processing the module.  We have
to enhance our dependencies a bit, but they've always been quite good.

This simplifies things quite a bit.

Parsing options like --verbose and --debug can be a pain.  You need to
have everything set up before invoking parse_args(), but that may be a
significant amount of work, for which you may want verbose or
debugging enabled.

Thus the concept of "early" args: you can nominate arguments to be
parse before anything else, using opt_early_parse().

Make a false return abort the iteration, not true.

The old way makes sense for search functions (true == I found it), but
other kinds of iteration are more common (brute force search is
probably dumb).

strset_set -> strset_add
strset_clear -> strset_del
strset_test -> strset_get
strset_destroy -> strset_clear

Testing code still using it.

With the type canary, it's unnecessary.  Though the implementation is
a bit more awkward since they longer map directly through to list_top/tail.