search: when polling the keyboard, consume all waiting keystrokes

When during searching we look at the keyboard, then don't look at
just the first keystroke, but check if there are more, and if so,
check each one until we find a Cancel -- or until all keystrokes
have been consumed and we can continue.

This fixes the first half of https://savannah.gnu.org/bugs/?47438.

src/search.c

@@ -292,10 +292,14 @@ int findnextstr(
 
 	    lastkbcheck = time(NULL);
 
-	    if (input && func_from_key(&input) == do_cancel) {
-		statusbar(_("Cancelled"));
-		disable_nodelay();
-		return -2;
+	    /* Consume all waiting keystrokes until a Cancel. */
+	    while (input) {
+		if (func_from_key(&input) == do_cancel) {
+		    statusbar(_("Cancelled"));
+		    disable_nodelay();
+		    return -2;
+		}
+		input = parse_kbinput(NULL);
 	    }
 
 	    if (++feedback > 0)