RFE: new file verify API and output format

[pmatilai]

The existing file verify output is unnecessarily cryptic and is non-extensible. While it's possible to add new letters to the output, doing so likely breaks scripts relying on the output, the output remains cryptic, and being limited to single letters means there's no way to report issues from plugins (think IMA, fsverity, selinux...). We'll need a new API for this.

Besides just flagging "there's an inconsistency here", rpm could also show the expected value, at least on verbose level (originally requested in RhBug:1750188. Both machine and human consumers should be considered.

Once the details are clear this will be turned into a ticket.