Get rid of a couple of copies of the CC0 text. Add header comment to
CC0 files to remind people editing them what the terms are. Also add
some missing XML headers. The header comments in layouts won't bloat
the binary - they get stripped out before compressing, same as any other
comments.
- Fix stupid logic errors in software_list_device::internal_validity_check
- Allow info and feature list entries to provide an empty string as the value
- Change a couple of null publisher entries in vz_cass.xml to "<unknown>"
