unix,win: add uv_fs_access()

@saghul here is the `uv_fs_access()` PR you asked for. The Unix side seems to be working, but on the Windows side, `GetFileAttributesW()` is always returning `INVALID_FILE_ATTRIBUTES`. Suggestions are more than welcome.
From the initial conversation, it sounded like it wasn't "exactly" equivalent to `access(2)` on windows, due to differing permissions models. For linux, you can RTM, but for windows, you have to read the libuv source so, it might be worth adding a comment here on what it does on windows.