The default_stat fn should be public

Anybody working with a C API that uses the stat structure will want `default_stat` to be accessible, since, as can be seen by the implementation of default_stat, all uses of `stat` without it would need to be platform-specific.

In this PR I have simply made it public--I would understand if there was a desire to put the public API somewhere else instead.  (There is no reason it should be in `std::path`.)  Maybe an impl of `Default` in `std::libc` itself?