Make fd_set fields public

[sfackler]

Closes #36

[thibault-ml]

I had an issue when using fd_set because of private fields. I ended up having to use malloc, which has isn't great.

However, I don't believe the field should be public as it has private meanings.
Instead, libc should provide a new method or a initialiser function.

[alexcrichton]

I'd be fine to merge something like this, but the CI builds are red unfortunately.

Note that in the meantime you can initialize an fd_set with mem::{uninitialized, zero} and then use the FD_* macros to manipulate them (as C would)

[alexcrichton]

I think this needs some more work to move forward (e.g. rather than just making the fields public), so I'm going to close this for now. Feel free to resubmit if the definitions are updated though!