Addition of clang nullability qualifiers.

Add two new qualifiers for use by the static checkers:

_Nonnull
The _Nonnull nullability qualifier indicates that null is not a meaningful
value for a value of the _Nonnull pointer type.

_Nullable
The _Nullable nullability qualifier indicates that a value of the
_Nullable pointer type can be null.

These were introduced in Clang 3.7. For more information, see:
http://clang.llvm.org/docs/AttributeReference.html#nonnull

We add these now without using them so that the GCC ports have time to
pick up the header change.

Hinted by:	Android Bionic libc [1]
Also seen in:	Apple's Libc-1158.20.4

[1]
baa2a973bd

sys/sys/cdefs.h

@@ -803,6 +803,14 @@
 #include <machine/acle-compat.h>
 #endif
 
+/*
+ * Nullability qualifiers: currently only supported by Clang.
+ */
+#if !(defined(__clang__) && __has_feature(nullability))
+#define	_Nonnull
+#define	_Nullable
+#endif
+
 /*
  * Type Safety Checking
  *