canput() searches through the stream (starting at q) until it finds a queue containing a service routine where the message can be enqueued, or until it reaches
the end of the stream. If found, the queue containing the service routine is tested to see if there is room for a message in the queue.
canputnext(q) and bcanputnext(q, pri) should always be used in preference to canput(q->q_next) and bcanput(q->q_next, pri) respectively.