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/server_test.c

@@ -287,7 +287,7 @@ server_connection_ownership(struct ct *t)
 	CT_CHECK(t, readreply(t, &a, 1, "", preedit));
 	CT_EQ_STR(t, "", preedit);
 	CT_EQ_PTR(t, bowner, testengineowner());
-	if(!sendkey(t, &b, 1, 0, Kmodfirst))
+	if(!sendkey(t, &b, 1, 0, 0))
 		goto cleanup;
 	req = nextrequest(t, &f.pump, Keypress);
 	allowrequest(&f.pump);