Update libc dependency

[CAD97]

You don't support libc@0.1. So don't say you do.

Yes, cargo by default will give you libc@0.2, and everything will work. But also, libc could release a version 0.3 with API changes (however unlikely that may be), and you would break because of that. And you don't even have the benefit of allowing a 1.0 libc, either.

You don't necessarily need to be fully minimal-versions correct (though it would be better if you were), but saying you work with libc@0.1 and libc@0.3 is just completely incorrect.