add file signature for cpio

[doug24]

cpio file format used in RPM Package Manger
File signature from https://www.garykessler.net/library/file_sigs.html

[squid-box]

Sorry for being slow to respond, trying to get through the backlog a little bit at a time...

Looking at the documentation you're linking, it seems you've omitted the last byte

The last part, nn, has three different valid values - have you tested against a cpio file with your changes?

[doug24]

No problem, thanks for reviewing it.

I have a fork of SevenZipSharp that I have been using for dnGrep, and this is the cpio signature I've been using in releases for two years. I originally used a longer signature (with 0x32). When I shortened it, by commit comment was "change signature for cpio files there are three variations in the sixth position, so match on just the first five".

I recently tested it with a Kibana rpm, which has a cpio archive in the rpm. I have not specifically tested this signature with all three formats (how to find them??), but I also haven't had any bug reports using this shorter one.

I'd like to drop my fork and change to use a maintained package, now that I see you have one. This is the only issue I found when changing to your NuGet package.