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.
style nit: prefer the following:
```c
int uv_fs_access(uv_loop_t* loop,
                 uv_fs_t* req,
                 const char* path,
                 int flags,
                 uv_fs_cb cb) {
```