mkdino.py: "extract-rpm" command, xz support, etc.

* "build": default compression is now XZ

We want to do apples-to-apples comparisons of the on-disk size, so we
should use the same compression algorithm as RPM.. at least for now.

* "build": added --compress, --compresslevel

You can switch back to zstd compression or tweak the compression level
if you like.

* improved "info" output

"info" will now tell you which compression algorithm is being used,
among other things.

* added "extract-rpm" command

Yes, you can get files back out! Hooray!

output is two files right now (rpm header and cpio payload) but this
will probably change once I figure out if RPM will accept packages with
uncompressed payloads...

* misc.

There's some improvements to info/list matching