fortify: Move remaining fortify helpers into fortify-string.h

When commit a28a6e86 ("string.h: move fortified functions definitions
in a dedicated header.") moved the fortify-specific code, some helpers
were left behind. Move the remaining fortify-specific helpers into
fortify-string.h so they're together where they're used. This requires
that any FORTIFY helper function prototypes be conditionally built to
avoid "no prototype" warnings. Additionally removes unused helpers.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Acked-by: Francis Laniel <laniel_francis@privacyrequired.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>