Avoid treating non-X GDK display names as X ones

When running on Wayland, gdk_display_get_name() can return things like
"wayland-0" rather than valid X display names.  PuTTY nonetheless
treated them as X display names, meaning that when running under
Wayland, pterm would set DISPLAY to "wayland-0" in subprocesses, and
PuTTY's X forwarding wouldn't work properly.

To fix this, places that call gdk_display_get_name() now only do so on
displays for which GDK_IS_X_DISPLAY() is true.  As with
GDK_IS_X_WINDOW(), this requires some backward-compatibility for GDK
versions where everything is implicitly running on X.

To make this work usefully, pterm now also won't unset DISPLAY if it
can't get an X display name and instead will pass through whatever value
of DISPLAY it received.  I think that's better behaviour anyway.

There are two separate parts of PuTTY that call gdk_display_get_name().
platform_get_x_display() in unix/putty.c is used for X forwarding, while
gtk_seat_get_x_display() in unix/window.c is used used for setting DISPLAY
and recording in utmp.  I've updated both of them.

unix/gtkcompat.h

@@ -187,6 +187,7 @@
 #endif /* 2.24 */
 
 #if !GTK_CHECK_VERSION(3,0,0)
+#define GDK_IS_X11_DISPLAY(display) (1)
 #define GDK_IS_X11_WINDOW(window) (1)
 #endif
 

unix/pty.c

@@ -1138,8 +1138,6 @@ Backend *pty_backend_create(
                 char *x_display_env_var = dupprintf("DISPLAY=%s", x_display);
                 putenv(x_display_env_var);
                 /* As above, we don't free this. */
-            } else {
-                unsetenv("DISPLAY");
             }
         }
 #endif

unix/putty.c

@@ -18,6 +18,10 @@
 
 #include "gtkcompat.h"
 
+#ifndef NOT_X_WINDOWS
+#include <gdk/gdkx.h>
+#endif
+
 /*
  * Stubs to avoid pty.c needing to be linked in.
  */
@@ -62,8 +66,11 @@ const bool dup_check_launchable = true;
 
 char *platform_get_x_display(void) {
     const char *display;
+#ifndef NOT_X_WINDOWS
     /* Try to take account of --display and what have you. */
-    if (!(display = gdk_get_display()))
+    if (!GDK_IS_X11_DISPLAY(gdk_display_get_default()) ||
+        !(display = gdk_get_display()))
+#endif
         /* fall back to traditional method */
         display = getenv("DISPLAY");
     return dupstr(display);

unix/window.c

@@ -411,8 +411,8 @@ StripCtrlChars *gtk_seat_stripctrl_new(
 static void gtk_seat_notify_remote_exit(Seat *seat);
 static void gtk_seat_update_specials_menu(Seat *seat);
 static void gtk_seat_set_busy_status(Seat *seat, BusyStatus status);
-static const char *gtk_seat_get_x_display(Seat *seat);
 #ifndef NOT_X_WINDOWS
+static const char *gtk_seat_get_x_display(Seat *seat);
 static bool gtk_seat_get_windowid(Seat *seat, long *id);
 #endif
 static void gtk_seat_set_trust_status(Seat *seat, bool trusted);
@@ -439,10 +439,11 @@ static const SeatVtable gtk_seat_vt = {
     .prompt_descriptions = gtk_seat_prompt_descriptions,
     .is_utf8 = gtk_seat_is_utf8,
     .echoedit_update = nullseat_echoedit_update,
-    .get_x_display = gtk_seat_get_x_display,
 #ifdef NOT_X_WINDOWS
+    .get_x_display = nullseat_get_x_display,
     .get_windowid = nullseat_get_windowid,
 #else
+    .get_x_display = gtk_seat_get_x_display,
     .get_windowid = gtk_seat_get_windowid,
 #endif
     .get_window_pixel_size = gtk_seat_get_window_pixel_size,
@@ -4348,12 +4349,14 @@ void modalfatalbox(const char *p, ...)
     exit(1);
 }
 
+#ifndef NOT_X_WINDOWS
 static const char *gtk_seat_get_x_display(Seat *seat)
 {
-    return gdk_get_display();
+    if (GDK_IS_X11_DISPLAY(gdk_display_get_default()))
+        return gdk_get_display();
+    return NULL;
 }
 
-#ifndef NOT_X_WINDOWS
 static bool gtk_seat_get_windowid(Seat *seat, long *id)
 {
     GtkFrontend *inst = container_of(seat, GtkFrontend, seat);