Adjusted the symlink reparse point detection code to be more specific.

The first and third bits, the 'microsoft' bit and 'name surrogate' bit respectively, are now checked as well. The docs aren't definitive on this, but this meshes up with the definition in winnt.h so it should be right.
Made function for identifying if a file is a symlink a public module function.

This is necessary so the file module can be used on *nix and Windows in the same way.