Merge branch...

Merge branch 'rnou/228-cat-and-other-system-commands-use-new-fstatat-with-at_empty_path-flag' into 'master'

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

See merge request !149