cloudabi-run: writeable files

[dckc]

I see a TODO for this; I'm opening this issue as a way of registering interest and subscribing to progress. I think I need it for https://github.com/dckc/ZeroV

  if (tag == "tag:nuxi.nl,2015:cloudabi/file") {
    // Open a file by pathname.
    // TODO(ed): Make mode and rights adjustable.

-- https://github.com/NuxiNL/cloudabi-utils/blob/master/src/cloudabi-run/yaml_file_descriptor_factory.cc#L74-L76