Added uv_fs_req_result method

This is single commit of the pull request #1306.

I added this method because it makes easier and safer on a c# libuv binding library to get the result of an fs request on a callback. Without this method I would have to replicate and then marshal the entire uv_fs_t structure. This is prone to errors and difficult to maintain: if you ever change the internals of uv_fs_t my library will break. As long as it's a really small change in libuv, can you please consider to merge this patch? I'm pretty much sure this will come useful to other binding libraries. Thanks