Rar support

[delcypher]

@limelime This looks like a very valuable contribution. Thanks! I've been meaning to write tests but I never really got around to it.

I want to test this a bit first but once I'm happy I'll go ahead and merge unless @arfoll has any objections.

[delcypher]

@arfoll @limelime I've just landed a bunch of changes to do much better testing. I opted to use lit rather than write tests directly in bash so that I can

• Run tests in parallel
• Selectively enable/disable tests depending on the testing environment. This is very useful if certain features don't work with a particular backend.

Your existing tests were a very handy guide on how to use rar so don't feel bad that I didn't use your tests.

Now that we have better testing this PR needs to be reworked to work with the new tests. Specifically to fit in with testing.

1. Modify tests/lit.site.cfg to be aware of the rar backend and fail if the rar binary is not available if that backend is requested. You will need to add a flag to unrarall similar to --7zip which forces the rar backend to be used.
2. Modify tests/run.sh to run lit -v --param unrarall_backend=rar.

So I guess the first step is to rework the core change (supporting rar as a backend) into its own commit and then add testing support.

[delcypher]

@arfoll @limelime Seeing as unrarall's code is fresh in my mind I thought I'd just implement rar support. I've done this in 3ea206b2e7ad7a5f9b83806f07f143e1cc5d0e01 .

@limelime Thank you for your work here. I'm closing this issue but if you find additional problems please file and issue or create a PR :)