os: Clear the -displayfd option after closing the file

Failing to clear this means that we'll attempt to write the display
number to a random file descriptor on subsequent X server generations.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jamey Sharp <jamey@minilop.net>

os/connection.c

@@ -356,6 +356,7 @@ NotifyParentProcess(void)
         write(displayfd, display, strlen(display));
         write(displayfd, "\n", 1);
         close(displayfd);
+        displayfd = -1;
     }
     if (RunFromSmartParent) {
         if (ParentProcess > 1) {