engine: the one capability is a flag

Cclientpreedit was a one-bit mask that every producer set as "want ?
Cclientpreedit : 0" and every consumer masked out again; nothing else
was ever going to join it.  Keyreq carries clientpre, an int that says
whether the client draws the preedit, and the engine and the XIM
context keep it under that name.

tests/ipc_test.c

@@ -70,7 +70,7 @@ void
 ipc_control_and_caret_frames(struct ct *t)
 {
 	static const uchar capon[Ipcreqsz] = {
-		Ipcext|Cclientpreedit, Ipcversion, Ipcopcap, 0, 0, 0,
+		Ipcext|Ipcreqwant, Ipcversion, Ipcopcap, 0, 0, 0,
 	};
 	static const uchar capoff[Ipcreqsz] = {
 		Ipcext, Ipcversion, Ipcopcap, 0, 0, 0,
@@ -89,7 +89,7 @@ ipc_control_and_caret_frames(struct ct *t)
 	int valid, want;
 	int32_t x, y, h;
 
-	ipcpackcap(buf, Cclientpreedit);
+	ipcpackcap(buf, 1);
 	CT_EQ_MEM(t, capon, buf, sizeof capon);
 	CT_EQ_INT(t, Ipccap, ipcreqtype(buf));
 	/* The probe remains a harmless key-zero request to an old daemon. */