Kernel+LibC: Add fstatat

The function fstatat can do the same thing as the stat and lstat
functions. However, it can be passed the file descriptor of a directory
which will be used when as the starting point for relative paths. This
is contrary to stat and lstat which use the current working directory as
the starting for relative paths.

Kernel/UnixTypes.h

@@ -680,6 +680,7 @@ struct rtentry {
 #define RTF_GATEWAY 0x2 /* the route is a gateway and not an end host */
 
 #define AT_FDCWD -100
+#define AT_SYMLINK_NOFOLLOW 0x100
 
 #define PURGE_ALL_VOLATILE 0x1
 #define PURGE_ALL_CLEAN_INODE 0x2