Don't parse FLTK arguments using Fl::arg; handle -display as other options

Fl::arg accepts several arguments which we are not documenting. Also,
it accepts a -geometry argument, which clashes with our own -geometry
option.
diff --git a/vncviewer/parameters.cxx b/vncviewer/parameters.cxx
index 39a7f07..d24e5bc 100644
--- a/vncviewer/parameters.cxx
+++ b/vncviewer/parameters.cxx
@@ -134,6 +134,9 @@
                           "Send the primary selection to the "
                           "server as well as the clipboard selection",
                           true);
+StringParameter display("display",
+			"Specifies the X display on which the VNC viewer window should appear.",
+			"");
 #endif
 
 StringParameter menuKey("MenuKey", "The key which brings up the popup menu",