waitforx changes

1) Changed alarm message from "Timed out waiting for RandR outputs" to
   "Timed out waiting for X display". The former was confusing, and the
   times where this message was triggered were nothing to do with
   RandR
2) Don't try to open a display of the form ":n" directory (n >= 0).
   If the X server hasn't yet opened its local socket, the
   XOpenDisplay() call can go to the network and possibly block
   for a long time. Instead, use the (undocumented) "unix:n" display
   specification, whih never goes to the network, and doesn't block.

waitforx/waitforx.c

@@ -2,6 +2,7 @@
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <ctype.h>
 #include <sys/signal.h>
 #include <unistd.h>
 
@@ -21,11 +22,32 @@ alarm_handler(int signal_num)
      *
      * Prefix the message with a newline in case another message
      * has been partly output */
-    const char msg[] = "\n<E>Timed out waiting for RandR outputs\n";
+    const char msg[] = "\n<E>Timed out waiting for X display\n";
     g_file_write(1, msg, g_strlen(msg));
     exit(XW_STATUS_TIMED_OUT);
 }
 
+/*****************************************************************************/
+/***
+ * Checks display is of the form ':n' where 'n' is an unsigned number
+ * @param display Display string
+ * @return boolean
+ */
+static int
+is_local_display(const char *display)
+{
+    int result = 0;
+    if (display != NULL && *display++ == ':')
+    {
+        while (isdigit(*display))
+        {
+            ++display;
+        }
+        result = (*display == '\0');
+    }
+    return result;
+}
+
 /*****************************************************************************/
 static Display *
 open_display(const char *display)
@@ -34,6 +56,23 @@ open_display(const char *display)
     unsigned int wait = ATTEMPTS;
     unsigned int n;
 
+    if (is_local_display(display))
+    {
+        // Don't use the raw display value, as this goes to the
+        // network if the X server port is not yet open. This can
+        // block if the network is configured in an unexpected way,
+        // which leads to use failing to detect the X server starting
+        // up shortly after.
+        //
+        // This code attempts to use a string such as "unix:10" to open
+        // the display. This is undocumented in the X11 man pages but
+        // is implemented in _xcb_open() from libxcb
+        // (which libX11 is now layered on).
+        char unix_display[64];
+        snprintf(unix_display, sizeof(unix_display), "unix%s", display);
+        return open_display(unix_display);
+    }
+
     for (n = 1; n <= ATTEMPTS; ++n)
     {
         printf("<D>Opening display %s. Attempt %u of %u\n", display, n, wait);