Allow a control message to contain multiple file descriptors (issue #566)

[Dretch]

ControlMessage has lost the Storable constraint, because it is not possible to implement Storable [Fd] because [Fd] is not fixed-size when encoded.

[kazu-yamamoto]

I'm using this code now to see what happens in my projects. Please wait for a while.

[andreasabel]

Looks like this PR broke the Windows build:

• https://github.com/haskell/network/issues/574