Returned error messages

When an error event occurs, the backend should send a message to the user.

Before sending a message, the backend should check the PIO_IPCWRITEFD environment variable. If it is set, the message is sent to a print supervisor by way of a pipe. The print supervisor interprets the message and sends it to the user. If the PIO_IPCWRITEFD environment variable is not set, the backend sends the message to the user with the sysnot routine.

The qdaemon print spooler always uses the _sysnot (Back-end routines in libqb) routine to send messages. Non-base operating system print spoolers can use the sysnot routine or the pipe to send messages.