add rcs support to cargo-new|init

[worr]

RCS is a venerable version control system available in many UNIX systems for local version tracking. Adds support for initializing an RCS repo and discovering it as an existing version control system.

---

I was starting a Rust project on an OpenBSD system that didn't have git available, and reached for my other typical local VCS, RCS. I thought it'd be nice if Cargo could make RCS repos automatically, and so added support for it.

Differing a bit from other VCSes, we need to init RCS slightly later in the lifetime of creating a repo, which is why I included a late_init function to complete the initialization after the package has already been laid out.

I tested creating several projects locally, and doing cargo init on existing projects without RCS repos.

[rustbot]

r? @weihanglo

rustbot has assigned @weihanglo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[weihanglo]

Thanks for being interested in this! I am sorry to tell you that this might be postponed at this moment 😞. Please read this comment for the team's thought on adding new VCS support. In short,

• We currently not accept adding any new VCS support.
• It's unclear what it means to “have VCS support built-in” in Cargo.
• We are interested in making VCS support more extensible, such as leveraging tooling from VCS itself or working on Cargo templates.

Also, per the contributor guide, it is encouraged to start from an issue first before submitting a pull request, so we can avoid the situation like this. Sorry again.

[weihanglo]

@worr would you mind turning this into an issue? And we'll can link them holistically.