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/API/Syscall.h

@@ -422,6 +422,7 @@ struct SC_waitid_params {
 };
 
 struct SC_stat_params {
+    int dirfd;
     StringArgument path;
     struct stat* statbuf;
     int follow_symlinks;