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.

tests/live.c

@@ -691,5 +691,5 @@ ipcprobe(int fd, char *where)
 {
 	unsigned char empty[] = {0, 0, 0, 0, 0};
 
-	return ipcrequest(fd, 0, Kmodfirst, empty, sizeof empty, where);
+	return ipcrequest(fd, 0, 0, empty, sizeof empty, where);
 }