Resolve "`cat` and other system commands use (new)fstatat with AT_EMPTY_PATH flag"

Before this MR: The flag is ignored in relativize_fd_path, but the man page shows that dirfd can be a file if the EMPTY_PATH is on. It seems that newer (kernel/coreutils/etc) uses that instead of doing a normal stat to the file.

Therefore, we need to handle empty path during relativize paths so that the file descriptor is used instead.

Closes #228 (closed)