mkdir: drop mkdir_errno_wrapper(), use mkdirat_errno_wrapper() instead

mkdir: drop mkdir_errno_wrapper(), use mkdirat_errno_wrapper() instead

Let's reduce our code duplication, and let's focus on using xyzat()
style APIs more, hence drop mkdir_errno_wrapper() and stick to
mkdirar_errno_wrapper() wherever we can, it's a true superset of
functionality after all.