RFE: add option to use an external decompressor to rpm2cpio and rpm2archive

[pmatilai]

As it is, at least rpm2cpio happens to allow an external decompressor for unsupported formats because rpmio is sloppy and just returns the compressed stream as such for any invalid/unknown decompressor flags, eg rpm2cpio foo.rpm | zstd -d | cpio can be used to work around lack of zstd support in older versions of librpm. This is an useful thing to have, but it should be an explicit option rather than lucky incident.

1. rpmio needs to return failure for invalid compressor flags
2. rpm2cpio (and rpm2archive) need to check for that failure and error out cleanly
3. rpm2cpio (and rpm2archive) should offer an option to let an external decompressor handle it, ie just not reopen the piggyback stream at all
4. as a part of this, rpm2cpio be ported to popt, finally

Inspired by https://bugzilla.redhat.com/show_bug.cgi?id=2058426

[dralley]

@pmatilai As a behavioral change, perhaps this is suitable for RPM v6? (as in v6 of the tool, not v6 of the format)

[pmatilai]

Uh? This would be just an extra option, not replace any existing functionality.

[dralley]

I read this issue as being "we should fail, but currently we ignore by default. Let's fail by default and provide an option to ignore"

Which would be a change to how it currently behaves in addition to adding a new option (to restore present behavior)

But I might have read that incorrectly

[pmatilai]

Oh, I forgot there were all these loosely related steps listed in the description.
Whatever happens in this space is not significant behavior changes by any measure.