Use definitions from libc instead of writing our own

[kamalmarhubi]

[kamalmarhubi]

I'm going to start going through module by module over the next week or so.

[kamalmarhubi]

Thinking about this a bit more, we may want our own analog of the sys module pattern. Right now it's hard to track down where items come in because the cfg attrs are all over the place. @arcnmx you have a recently opened RFC on a related topic. Thoughts?

[kamalmarhubi]

Ah, discussion of conditional compilation module pattern would belong in #98.