Allow apply_default_acl() to take a stat pointer for its path argument.

When operating recursively, nftw has already called stat() on the path
that it feeds to apply_default_acl(). Rather than re-stat it, we can
now pass a stat struct pointer in as the second argument to
apply_default_acl(). When not operating recursively, you just pass it
NULL instead, and apply_default_acl() will do the fstat as before.