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putbq() places a message at the beginning of the appropriate section of the message queue. There
are always sections for high priority and ordinary messages. If other priority bands are used, each will have its own section of the queue, in priority band order, after high priority messages and before ordinary messages. putbq() can be used for ordinary, priority band, and high priority
messages. However, unless precautions are taken, using putbq() with a high priority message is likely to lead to an infinite loop of putting the message back on the queue, being rescheduled, pulling it off, and putting it back on.
This function is usually called when bcanput(9F) or canput(9F) determines
that the message cannot be passed on to the next stream component. The flow control parameters are updated to reflect the change in the queue's status. If QNOENB is not set, the service routine is enabled.
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