Supporting unrar introduces licensing issues

[cyqsimon]

The underlying C++ library of unrar.rs uses a proprietary license that includes some hostile restrictions. As explained by Fedora Wiki, these restrictions make it GPL-incompatible and non-free.

• The good news is that ouch does not distribute unrar sources directly, which means it's not subjected to the requirements of term 2, which means it can continue to be licensed under MIT license (I think; obligatory "not a lawyer").
• The bad news is that building ouch would still require non-free code, which makes it legally impossible to include it in FOSS-only distributions like Debian and Fedora, or to build it using third-party build services with FOSS-only TOS (namely Fedora COPR).

As I see it, there are two ways to solve this problem:

1. Feature-gate unrar support
2. Migrate to a free unrar library (e.g. unrar-free, which is shipped by Fedora)

Of course, the two are not mutually exclusive. I will submit a PR for the feature-gate first.