fortify: Prepare to improve strnlen() and strlen() warnings

In order to have strlen() use fortified strnlen() internally, swap their
positions in the source. Doing this as part of later changes makes
review difficult, so reoroder it here; no code changes.

Cc: Francis Laniel <laniel_francis@privacyrequired.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>