Define stable cross-platform file open constants so that users can
pass `UV_FS_O_RDWR` rather than `_O_RDWR` (win) or `O_RDWR` (unix).
Map `UV_FS_O_DIRECT`, `UV_FS_O_DSYNC` and `UV_FS_O_SYNC` to
`FILE_FLAG_NO_BUFFERING` and `FILE_FLAG_WRITE_THROUGH` (win).
Fixes: https://github.com/libuv/libuv/issues/1550
PR-URL: https://github.com/libuv/libuv/pull/1567
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>