ipc: share one keysym and modifier mapping across frontends

The keysym-to-engine-key rule lived in ibus.c, xim/xim.c and gtk/main.c,
and the copies disagreed: GTK sent keypad digits as special keys while
IBus and XIM sent '1'..'9'. Modifiers were rebuilt bit by bit in two
places although Mmask already is the X core layout; only the virtual
Super bit (26) that GDK and IBus set needs folding. ipckey/ipcmod in
ipc.c serve the daemon, the module and the tests.

ipc.h

@@ -38,11 +38,13 @@ enum
 	Kmodfirst = Kspec|0xe1,
 	Kmodlast = Kspec|0xee,
 
+	/* X core modifier bits; GDK and IBus also flag Super at bit 26. */
 	Mshift = 1<<0,
 	Mctrl = 1<<2,
 	Malt = 1<<3,
 	Msuper = 1<<6,
 	Mmask = Mshift|Mctrl|Malt|Msuper,
+	Msupervirt = 1<<26,
 
 	Ipcreqsz = 6,
 	Ipccaretsz = 16,
@@ -70,6 +72,8 @@ struct Ipcresp
 	size_t	preeditlen;
 };
 
+uint32_t	ipckey(uint32_t, uint32_t);
+uint32_t	ipcmod(uint32_t);
 void	ipcpackreq(unsigned char[Ipcreqsz], int, uint32_t, uint32_t);
 void	ipcpackreset(unsigned char[Ipcreqsz], int);
 void	ipcpackcap(unsigned char[Ipcreqsz], int);