Added new cupsBackchannelRead() and cupsBackchannelWrite()

Added new cupsBackchannelRead() and cupsBackchannelWrite()
functions, as well as backchannel support to the parallel,
serial, socket, and USB backends (STR #1252)

The parallel and USB backends now treat a "no space available"
error as an out-of-paper condition (STR #1225)

backend/ipp.c:
    - report_printer_state(): Cleanup printer status handling.

backend/parallel.c:
    - main(): Add backchannel support, and check for ENOSPC
      error.

backend/serial.c:
    - main(): Add backchannel support.

backend/socket.c:
    - main(): Add backchannel support.

backend/usb-unix.c:
    - print_device(): Add backchannel support, and check for
      ENOSPC error.

cups/backchannel.c:
    - Add cupsBackchannelRead() and cupsBackchannelWrite() functions.

cups/cups.h:
    - Add cupsBackchannelRead() and cupsBackchannelWrite() functions.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@4658 7a7537e8-13f0-0310-91df-b6672ffda945