Implement an fdatasync alternative on OS X

[argiopetech]

fdatasync() is not supported (as such) on OS X. An alternative using fcntl() is detailed in a picoc issue.

We should provide the fcntl()-based alternative on OS X (and presumably iOS).

[hvr]

Even though this may appear convenient at first, I'm very sceptical about making unix into an emulation layer. I'd rather have unix's goal be focusing on providing API bindings for system calls provided by the underlying OS, unless there's a generic and portable fallback implementation possible that accurately implements the required semantics.

I'm strongly against starting to clutter the code with all sorts of #if darwin_HOST_OS conditionals, as this violates Autoconf's philosophy of checking for features rather than os/versions.

[hasufell]

fcntl is documented by posix: https://pubs.opengroup.org/onlinepubs/007904975/functions/fcntl.html

We seem to have a module for it: https://github.com/haskell/unix/blob/master/System/Posix/Fcntl.hsc

But no binding to it.

The main caveat is that fcntl is a variadic function. With the capi calling convention we can interface with such functions, but still have to give them a constant number of arguments: https://downloads.haskell.org/~ghc/8.10.7/docs/html/users_guide/ffi-chap.html?highlight=capi#varargs-not-supported-by-ccall-calling-convention

So I'm not sure how useful a raw binding would be here.

My guess would be someone can implement an alternative to fdatasync in a new library?