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/ipc_test.c

@@ -60,6 +60,7 @@ ipc_masks_modifiers(struct ct *t)
 	CT_EQ_UINT(t, Kspec + 0xff, ipckeysym(0xffff, 0x7f));	/* Delete */
 	CT_EQ_UINT(t, 0x1f642, ipckeysym(0x101f642, 0x1f642));
 	CT_EQ_UINT(t, 0, ipckeysym(0xfe03, 0));	/* ISO_Level3_Shift */
+	CT_EQ_UINT(t, 0, ipckeysym(0xffe1, 0));	/* Shift_L */
 	CT_EQ_UINT(t, Mshift|Mctrl, ipcmod(Mshift|Mctrl|(1<<1)|(1<<4)));
 	CT_EQ_UINT(t, Msuper, ipcmod(1<<6));
 	CT_EQ_UINT(t, Msuper, ipcmod(1<<26));