Add a new __sentinel attribute.

Add a new __sentinel attribute.

The sentinel attribute was originally implemented in OpenBSD's gcc and
later adopted by upstream GCC 4.0 (and clang). From the OpenBSD's
gcc-local manpage:

-   gcc recognizes the extra attribute __sentinel__, which can be used to
    mark varargs function that need a NULL pointer to mark argument
    termination, like execl(3).  This exposes latent bugs for 64-bit
    architectures, where a terminating 0 will expand to a 32-bit int, and
    not a full-fledged 64-bits pointer.

While here sort the visibility attributes.

Hinted-by:	OpenBSD