Lock access to the twin of a qid more carefully. Previously, the code

Lock access to the twin of a qid more carefully.  Previously, the code
for sending messages was not locked at all, which meant that there
were race conditions that could produce fatal errors.  By locking, we
limit the amount of concurrent communication slightly, but since the
locks are active for only a short time this shouldn't be noticed.

Also implement a new procedure OS2_qid_tqueue, which is used by
inferior threads to obtain and close their associated tqueue objects
when they finish.