There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>

This prevents theoretical resource leak, since those threads are never joined.

Found with ThreadSanitizer.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

The ngx_thread_pool_done object isn't volatile, and at least some
compilers assume that it is permitted to reorder modifications of
volatile and non-volatile objects.  Added appropriate ngx_memory_barrier()
calls to make sure all modifications will happen before the lock is released.

Reported by Mindaugas Rasiukevicius,
http://mailman.nginx.org/pipermail/nginx-devel/2016-April/008160.html.

No functional changes.

It's not needed for completed tasks queue since the previous change.

No functional changes.

Work around pthread_cond_destroy() and pthread_mutex_destroy() returning
EBUSY.  A proper solution would be to ensure all threads are terminated.

Behave like POSIX semaphores.  If N worker threads are waiting for tasks,
at least that number of tasks should be allowed to be put into the queue.

It's not needed for completed tasks queue.

No functional changes.