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.

ipc.c

@@ -165,15 +165,17 @@ ipcconnect(void)
 
 /*
  * Engine key for an X keysym and its Unicode value: printable characters
- * as themselves, the function keysyms 0xff00-0xffff as Kspec+offset.
- * The keypad keys and Shift+Tab (ISO_Left_Tab) fold onto their plain
- * counterparts.
+ * as themselves, the function keysyms 0xff00-0xffff as Kspec+offset, and
+ * no key at all for a modifier.  The keypad keys and Shift+Tab
+ * (ISO_Left_Tab) fold onto their plain counterparts.
  */
 uint32_t
 ipckeysym(uint32_t sym, uint32_t unicode)
 {
 	if(unicode >= ' ' && unicode != 0x7f)
 		return unicode;
+	if(sym >= 0xffe1 && sym <= 0xffee)
+		return 0;
 	switch(sym){
 	case 0xfe20:
 	case 0xff89: