Optional features are not build tested

[alex-gulyas]

Currently none of the available optional features are built by Travis, therefore:

• patches for the optional parts are not tested by Travis (or Homu)
• patches for other parts can silently break the optional parts (see Build fails with execvpe feature enabled #306, Build fails with signalfd feature enabled #307)

[kamalmarhubi]

Thanks for bringing this up!

I think the right direction will be to break out the optional parts into separate crates. Eg, for signalfd we would want the function in nix-core but the iterator in nix-signalfd (or something like that).

This is still a little undecided. #190 is related.

[Susurrus]

Why don't we just enable the features in testing? I don't think this is a good rationale for splitting nix into several separate crates.