This PR splits `internals.rs` into the `internal` module with multiple
files, and moves `FdOptions` into `opts.rs`.
The main motivation behind this is to move logic for constructing
`FdOptions` out of the main function and more readable and easier to
understand in the `opts` module. The goal will eventually to be able
to write `FdOptions::from(matches)` and have the options constructed.
Josh Leeb-du Toit