docs: stop suggesting strlcpy

Since strlcpy is deprecated, the documentation shouldn't suggest using
it. This patch fixes the examples to use strscpy instead. It also uses
sizeof instead of underlying constants as far as possible, to simplify
future changes to the corresponding data structures.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>