enable/disable O_DIRECT

[thomasjungblut]

Hey there,

thanks for this awesome library! I was playing around with this today a bit.

I do have a somewhat silly requirement in that some writes may not be aligned with the block size (forced flushes/fsyncs out of my control). On Linux it's fairly easy to toggle O_DIRECT on and off through fcntl, I assume on MAC it works the same using F_NOCACHE.

Now I'm currently browsing the win APIs, but I don't find anything similar. Do you have a clue?

Would you be ok if I'd send you a PR for such a feature?

[ncw]

Sorry for the delay in responding!

If you wanted to send a PR like that it would be fine with me, however it would need to work on all platforms or possibly give an error if not supported. Any idea about Windows support?