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.

Userland/Libraries/LibC/fcntl.h

@@ -40,6 +40,7 @@ __BEGIN_DECLS
 int creat(const char* path, mode_t);
 int open(const char* path, int options, ...);
 #define AT_FDCWD -100
+#define AT_SYMLINK_NOFOLLOW 0x100
 int openat(int dirfd, const char* path, int options, ...);
 
 int fcntl(int fd, int cmd, ...);