Replace all instances of `c_int` type annotations with appropriate type alias

[khionu]

This will help for correctness and readability. I haven't looked into where existing type aliases may be, but there are some number of them in std, eg std::os::fd::RawFd.

[khionu]

Note, the other Fd wrappers have gatchas, so we should either provide our own wrappers around RawFd or stick with just RawFd for now.

[pat-glitch]

Hi, I would like to start contributing, and I want to work on this issue! In what file/files do I need to replace the c_int type innotations?
/assign