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

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>