Add mechanism to "unread" messages back to the head of a message

Add mechanism to "unread" messages back to the head of a message
queue.  Add mechanism to get the other end of a qid pair.  Eliminate
logic error that was being generated when sending a message to a qid
whose other end had been closed (now we just discard the message).
Reimplement message queues to NOT use the OS/2 queue abstraction --
apparently these queues use quite alot of memory and I recently ran
into a situation in which the memory was exhausted, which caused
Scheme to die.  Now, the memory needed for the queues is taken from
the Scheme process's virtual memory.