ipc: a modifier keysym is no key

Shift, Control, and their kin were mapped to special keys that the
engine then had a range and a check to ignore, and the GTK module made a
round trip to the daemon for each press.  ipckeysym maps them to key 0,
which was already the "no key" every frontend and the engine skip;
Kmodfirst, Kmodlast, and ismodkey go.

strans.c

@@ -516,17 +516,11 @@ kokey(Rune c, u32int mod)
 	return c;
 }
 
-static int
-ismodkey(u32int ks)
-{
-	return ks >= Kmodfirst && ks <= Kmodlast;
-}
-
-/* Modifier presses and unmapped keys never engage the engine. */
+/* Modifier presses and unmapped keys, key 0, never engage the engine. */
 int
 keymeaningful(u32int ks)
 {
-	return ks != 0 && !ismodkey(ks);
+	return ks != 0;
 }
 
 static void