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

sys/sys/cdefs.h

@@ -469,11 +469,13 @@
 #endif
 
 #if __GNUC_PREREQ__(4, 0)
-#define	__hidden	__attribute__((__visibility__("hidden")))
+#define	__sentinel	__attribute__((__sentinel__))
 #define	__exported	__attribute__((__visibility__("default")))
+#define	__hidden	__attribute__((__visibility__("hidden")))
 #else
-#define	__hidden
+#define	__sentinel
 #define	__exported
+#define	__hidden
 #endif
 
 /*