Correctly detect if DISPLAY env var is set but specified display is not existing or xserver not running, and start in text mode

src/main.c

@@ -71,6 +71,7 @@ static void child_handler(int signum)
 int main(int argc, char **argv)
 {
   int i;
+  Display *display;
   signal(SIGINT, sig_handler);
   signal(SIGSEGV, sig_handler);
   signal(SIGILL, sig_handler);
@@ -82,6 +83,16 @@ int main(int argc, char **argv)
   /* 20181013 check for empty or non existing DISPLAY *before* calling Tk_Main or Tcl_Main */
 #ifdef __unix__
   if(!getenv("DISPLAY") || !getenv("DISPLAY")[0]) has_x=0;
+  else {
+    display = XOpenDisplay(NULL);
+    if(!display) {
+      has_x=0;
+      fprintf(errfp, "\n   X server connection failed, although DISPLAY shell variable is set.\n"
+                     "   A possible reason is that the X server is not running or DISPLAY shell variable\n"
+                     "   is incorrectly set.\n"
+                     "   Starting Xschem in text only mode.\n\n");
+    } else XCloseDisplay(display);
+  }
 #endif
   argc = process_options(argc, argv);
   my_strdup(_ALLOC_ID_, &xschem_executable, argv[0]);

src/xinit.c

@@ -2151,10 +2151,10 @@ int Tcl_AppInit(Tcl_Interp *inter)
   my_strncpy(home_dir, home_buff, S(home_dir));
 #endif
  /* set error and exit handlers */
- XSetErrorHandler(err);
  if(!interp) interp=inter;
  Tcl_Init(interp);
  if(has_x) {
+   XSetErrorHandler(err);
    Tk_Init(interp);
    tclsetvar("has_x","1");
  }