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