lightdm & xserver service: add ability to spawn more than one X server

- if xserver.tty and/or display are set to null, then don't specify
  them, or the -logfile argument in the xserverArgs
- For lightdm, we set default tty and display to null and we determine
  those at runtime based on arguments passed. This is necessary because
  we run multiple X servers so they can't all be on the same display

nixos/modules/services/x11/display-managers/default.nix

@@ -37,7 +37,7 @@ let
   # file provided by services.xserver.displayManager.session.script
   xsession = wm: dm: pkgs.writeScript "xsession"
     ''
-      #! /bin/sh
+      #! ${pkgs.bash}/bin/bash
 
       . /etc/profile
       cd "$HOME"

nixos/modules/services/x11/display-managers/lightdm.nix

@@ -13,9 +13,16 @@ let
   # lightdm runs with clearenv(), but we need a few things in the enviornment for X to startup
   xserverWrapper = writeScript "xserver-wrapper"
     ''
-      #! /bin/sh
+      #! ${pkgs.bash}/bin/bash
       ${concatMapStrings (n: "export ${n}=\"${getAttr n xEnv}\"\n") (attrNames xEnv)}
-      exec ${dmcfg.xserverBin} ${dmcfg.xserverArgs}
+
+      display=$(echo "$@" | xargs -n 1 | grep -P ^:\\d\$ | head -n 1 | sed s/^://)
+      if [ -z "$display" ]
+      then additionalArgs=":0 -logfile /var/log/X.0.log"
+      else additionalArgs="-logfile /var/log/X.$display.log"
+      fi
+
+      exec ${dmcfg.xserverBin} ${dmcfg.xserverArgs} $additionalArgs "$@"
     '';
 
   usersConf = writeText "users.conf"
@@ -39,7 +46,6 @@ let
       greeter-session = ${cfg.greeter.name}
       ${cfg.extraSeatDefaults}
     '';
-
 in
 {
   # Note: the order in which lightdm greeter modules are imported
@@ -98,7 +104,6 @@ in
   };
 
   config = mkIf cfg.enable {
-
     services.xserver.displayManager.slim.enable = false;
 
     services.xserver.displayManager.job = {
@@ -149,5 +154,7 @@ in
 
     services.xserver.displayManager.lightdm.background = mkDefault "${pkgs.nixos-artwork}/share/artwork/gnome/Gnome_Dark.png";
 
+    services.xserver.tty     = null; # We might start multiple X servers so let the tty increment themselves..
+    services.xserver.display = null; # We specify our own display (and logfile) in xserver-wrapper up there
   };
 }

nixos/modules/services/x11/xserver.nix

@@ -381,13 +381,13 @@ in
       };
 
       tty = mkOption {
-        type = types.int;
+        type = types.nullOr types.int;
         default = 7;
         description = "Virtual console for the X server.";
       };
 
       display = mkOption {
-        type = types.int;
+        type = types.nullOr types.int;
         default = 0;
         description = "Display number for the X server.";
       };
@@ -517,11 +517,12 @@ in
     services.xserver.displayManager.xserverArgs =
       [ "-ac"
         "-terminate"
-        "-logfile" "/var/log/X.${toString cfg.display}.log"
         "-config ${configFile}"
-        ":${toString cfg.display}" "vt${toString cfg.tty}"
         "-xkbdir" "${pkgs.xkeyboard_config}/etc/X11/xkb"
-      ] ++ optional (!cfg.enableTCP) "-nolisten tcp";
+      ] ++ optional (cfg.display != null) ":${tostring cfg.display}"
+        ++ optional (cfg.tty     != null) "vt${toString cfg.tty}"
+        ++ optional (cfg.display != null) [ "-logfile" "/var/log/X.${toString cfg.display}.log" ]
+        ++ optional (!cfg.enableTCP) "-nolisten tcp";
 
     services.xserver.modules =
       concatLists (catAttrs "modules" cfg.drivers) ++