Add print server for label printer to have it easily usable via SSH

Add and configure a print server for the Brother P-touch QL 500 label
printer, so that it can be easily used via SSH.

Do the following to make that work:
- Configure the print server host.
- Package printer-driver-ptouch to have a working driver for the label
  printer.
- Configure CUPS.
- Add a script "forcecommand-lpr-wrapper", which works together with the
  ForceCommand sshd_config option and wraps lpr to provide an easy
  interface to use the Brother QL 500 label printer via SSH.
- Add a print user and configure SSH to have the
  "forcecommand-lpr-wrapper" script accessible without a password using
  the print user via SSH.

config/hosts/ptouch-print-server/forcecommand-lpr-wrapper/forcecommand-lpr-wrapper.py

@@ -0,0 +1,84 @@
+#!/usr/bin/env python3
+
+# A script for usage with the ForceCommand sshd_config option.
+# It calls lpr with some standard arguments, but also parses
+# SSH_ORIGINAL_COMMAND to potentially provide a different set of arguments to
+# lpr.
+#
+# This wrapper is written for interacting with the Brother QL 500 label printer.
+#
+# The following options can be provided as an SSH command and this script will
+# then pass them to the lpr call: <MediaType> <PageSize>
+# - MediaType can be one of:
+#   - Labels
+#   - Tape (this is the default)
+# - PageSize can be one of:
+#   - 12mm
+#   - 12mm-circular
+#   - 17x54mm
+#   - 17x87mm
+#   - 23x23mm
+#   - 24mm-circular
+#   - 29mm
+#   - 29x90mm
+#   - 38mm
+#   - 38x90mm
+#   - 50mm
+#   - 54mm
+#   - 58mm-circular
+#   - 62mm
+#   - 62x29mm
+#   - 62x100mm
+#   - Custom.WIDTHxHEIGHT (with WIDTH and HEIGHT needing to be either one to
+#     three digits)
+#   - label-wide (this being a convenience alias for Custom.62x35mm and it also
+#     being the default)
+#   - label-item (this being a convenience alias for 38x90mm)
+# 
+# So using these options in a complete setup would look like this for example:
+# cat label-item.pdf | ssh print@ptouch-print-server.z9.ccchh.net labels label-item
+# This being equivalent to:
+# cat label-item.pdf | ssh print@ptouch-print-server.z9.ccchh.net Labels 38x90mm
+#
+# The options are case-insensitive.
+#
+# The options are derived from: lpoptions -p Brother-QL-500 -l
+
+import os, re, subprocess
+
+mediaType = "Tape"
+pageSize = "Custom.62x35mm"
+
+def parseGivenOptions():
+    givenOptionsString = os.environ["SSH_ORIGINAL_COMMAND"]
+    givenOptionsIterator = iter(givenOptionsString.split(" "))
+
+    givenMediaType = next(givenOptionsIterator, "")
+    givenPageSize = next(givenOptionsIterator, "")
+
+    global mediaType
+    if givenMediaType.lower() == "labels":
+        mediaType = "Labels"
+    elif givenMediaType.lower() == "tape":
+        mediaType = "Tape"
+
+    global pageSize
+    pageSizeRegex = re.compile(r"^((12mm(-circular)?)|(24mm-circular)|(58mm-circular)|(((17x(54|87))|(23x23)|((29|38)(x90)?)|(62x(29|100))|50|54|62)mm))$", re.ASCII | re.IGNORECASE)
+    pageSizeMatch = pageSizeRegex.match(givenPageSize)
+    pageSizeCustomRegex = re.compile(r"^custom\.(\d{1,3})x(\d{1,3})$", re.ASCII | re.IGNORECASE)
+    pageSizeCustomMatch = pageSizeCustomRegex.match(givenPageSize)
+    if givenPageSize.lower() == "label-wide":
+        pageSize = "Custom.62x35mm"
+    elif givenPageSize.lower() == "label-item":
+        pageSize = "38x90mm"
+    elif pageSizeMatch:
+        pageSize = givenPageSize.lower()
+    elif pageSizeCustomMatch:
+        width = pageSizeCustomMatch.group(1)
+        height = pageSizeCustomMatch.group(2)
+        pageSize = "Custom.{}x{}".format(width, height)
+
+if "SSH_ORIGINAL_COMMAND" in os.environ:
+    parseGivenOptions()
+
+subprocess.run(["lpr", "-P", "Brother-QL-500", "-o", "MediaType={}".format(mediaType), "-o", "PageSize={}".format(pageSize)])