thread-test: returned value of posix_thread_create() ignored if detached thread.
thread-test: randomly shuffle list of blocks to be freed remotely, with -Xr.
Make sure discovered threads are tagged as "running" for the xmalloc() layer.

This matters for the thread-private block allocation layer, since then a free
of some block from a foreign thread needs to be carefully handled and cannot
just be blindly re-inserted in the thread-private chunk: if the thread is live,
we would introduce race conditions leading to corruption.