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.

sesman/chansrv/rail.c

@@ -44,7 +44,7 @@
 #include "list.h"
 
 extern int g_rail_chan_id;      /* in chansrv.c */
-extern int g_display_num;       /* in chansrv.c */
+extern char g_display_str[];    /* in chansrv.c */
 extern char *g_exec_name;       /* in chansrv.c */
 extern tbus g_exec_event;       /* in chansrv.c */
 extern tbus g_exec_mutex;       /* in chansrv.c */