Add CaFormatExcludeFile flag to catar archives

[folbricht]

This adds CaFormatExcludeFile to feature flags as per #114. The catar flags produced for both are now: 0xb000000010001f22.

[tomberek]

Not sure if this should be added if the feature doesn't actually exist in desync. Or does it?

[folbricht]

I'm not even sure what that flag exactly means and why it's added to archives by casync.

[tomberek]

It it similar to .gitignore. Allows you to exclude files from a directory tree. See documentation at the end of https://github.com/systemd/casync/blob/master/doc/casync.rst

(at least i presume that the flag refers to this)

[folbricht]

That flag seems to refer more to a capability of the tool that produced the archive, not something the archive itself needs to know. casync adds that even without actually excluding files. So no, desync doesn't currently support this, but I'm ok adding it if that aligns better with casync.

[tomberek]

Let's leave the issue open to add the capability in the future, but merge this to get closer for reproducibility/testing?