Replace X11 display number with a display string

As far as possible, use of the X11 display number is kept to
X11-specific routines. This is to make it easier to restructure
the code to add non-X11 display support.

common/xrdp_constants.h

@@ -49,6 +49,12 @@
  **/
 #define MAX_XRDP_INSTANCE_NAMELEN 80
 
+/**
+ * Max length of buffer containing a Wayland or X11 display
+ * name
+ **/
+#define MAX_DISPLAY_NAME_SIZE 32
+
 /**
  * Size of buffer including terminator for a socket description, as
  * returned by g_sck_get_peer_description()