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/networking.nix

@@ -0,0 +1,23 @@
+{ ... }:
+
+{
+  networking = {
+    interfaces.net0 = {
+      ipv4.addresses = [
+        {
+          address = "10.31.208.13";
+          prefixLength = 25;
+        }
+      ];
+    };
+    defaultGateway = "10.31.208.1";
+    nameservers = [
+      "10.31.208.1"
+    ];
+  };
+
+  systemd.network.links."10-net0" = {
+    matchConfig.MACAddress = "BC:24:11:F2:CF:8F";
+    linkConfig.Name = "net0";
+  };
+}