Add Trusty OS support

[randomPoison]

This PR adds support for the Trusty secure operating system. This upstreams the patch that we have been using internally, cleaned up to meet the project's coding conventions.

This is a revival of #2987.

[rustbot]

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[randomPoison]

Addressing a comment from the original PR, @JohnTitor asked:

I wonder if we could move the module under linux_like or somewhere else similar to that. I'm not familiar with Trusty but is it completely different from Linux-like OS (i.e. exposing linux-like items on Trusty is harmful)?

Yeah, Trusty is not linux-like, nor is it a Unix. It has some overlap with those platforms, but isn't part of the same family of systems. I think the right thing to do is have Trusty be its own module.

[randomPoison]

I think this would also be a good candidate for merging over to the 0.2 branch. The diff should apply cleanly, I've tested it against both branches.

@rustbot label stable-nominated

[randomPoison]

What should I do about the unknown target_os = "trusty" cfg in CI? The targets aren't yet in the stable compiler. I've got this Cargo.toml snippet I've been using locally

[lints.rust.unexpected_cfgs]
level = "warn"
check-cfg = [
    'cfg(target_os, values("trusty"))',
]

Is that appropriate to add in the PR as a temporary way to suppress the lint? Or is there a better way to handle this?