GitLab

* Reimplementing destroyLocked as death test
* Comments
* Includes to use < > instead of " "
* Don't use c99-isms.

Because of fear of possible future collisions.

It can be used to check the thing is locked.

The test may be broken by design. In such case, it should be removed.
But it is left there for now, maybe it's just broken implementation and
someone (the reviewer?) may have an idea how to fix it.

And fix the tests. Some of them were broken a lot, some of them only
little.

Created with minimalistic set of functionality. Created a new library
for it, there'll be more thread-related stuff here.