lib/string/strchr/: stp[c]spn(), strrspn(), strnul(): Add macros and functions

Often, a pointer is more useful than a length when calling these.

Link: <https://docs.oracle.com/cd/E86824_01/html/E54769/strrspn-3gen.html>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/string/strchr/stpcspn.c

@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include <config.h>
+
+#include "string/strchr/stpcspn.h"