alter conf_grab(_kbd) to first ungrab AnyKey/AnyModifier, then proceed

to grab keys in keybindingq.  we don't need to ungrab/grab on every
addition to the queue, just once with a complete keybindingq; simplify
grabbing keys per screen (during init) and during a MappingNotify.

while here, change conf_grab_{kbd,mouse} to require only a Window.

client.c

@@ -213,7 +213,7 @@ client_setactive(struct client_ctx *cc, int fg)
 		XInstallColormap(X_Dpy, cc->colormap);
 		XSetInputFocus(X_Dpy, cc->win,
 		    RevertToPointerRoot, CurrentTime);
-		conf_grab_mouse(cc);
+		conf_grab_mouse(cc->win);
 		/*
 		 * If we're in the middle of alt-tabbing, don't change
 		 * the order please.