lib/string/zustr2stp.[ch]: Remove zustr2stp(); keep ZUSTR2STP()

The function should never be used; it's always used via its wrapper
macro.  To simplify, and reduce chances of confusion: remove the
function, and implement the macro directly in terms of
stpcpy(mempcpy(strnlen())).

Update the documentation, and improve the example, which was rather
confusing.

Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/string/zustr2stp.c

@@ -1,17 +1,7 @@
-/*
- * SPDX-FileCopyrightText: 2022-2023, Alejandro Colomar <alx@kernel.org>
- * SPDX-License-Identifier: BSD-3-Clause
- */
+// SPDX-FileCopyrightText: 2022-2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
 
 
 #include <config.h>
 
-#include <stddef.h>
-
-#ident "$Id$"
-
 #include "string/zustr2stp.h"
-
-
-extern inline char *zustr2stp(char *restrict dst, const char *restrict src,
-    size_t sz);