Eliminate the apply_default_acl_ex() function.

The apply_default_acl_ex() function was an "extended" version of the
apply_default_acl() function that, in addition, took a stat structure
pointer to the target path. The extended function was used by nftw(),
which usually has such a stat structure handy. However, the provenance
of that stat structure is not clear: does nftw() obtain it in a safe
way?

Since I don't know the answer to that question, I would rather stat()
the descriptor that I know was obtained safely. Thus there's no reason
to take the pointer as an argument, and then no reason to keep the
extended function around at all.