Show extra options in vncviewer --help

FLTK has some options (that we've also documented in the man page)
that should be mentioned in the --help output.

vncviewer/vncviewer.cxx

@@ -361,6 +361,14 @@ static void usage(const char *programName)
  programName,
 #endif
  programName, programName);
+
+#if !defined(WIN32) && !defined(__APPLE__)
+ fprintf(stderr,"\n"
+ "Options:\n\n"
+ " -display Xdisplay - Specifies the X display for the viewer window\n"
+ " -geometry geometry - Standard X position and sizing specification.\n");
+#endif
+
  fprintf(stderr,"\n"
  "Parameters can be turned on with -<param> or off with -<param>=0\n"
  "Parameters which take a value can be specified as "