Attempt to produce a DEP-5 machine-readable debian/copyright

Attempt to produce a DEP-5 machine-readable debian/copyright

I've treated section 55 of CDPA as though it's a licence, which it
kind of is.  I assume that quoting a single section of legislation
doesn't then require me to provide a copy of the Open Government
Licence as well.

Debian Policy section 2.3 says that a copyright file needs to contain
copyright statements only if the licence requires such statements.
CC0 does not, and so I've left them out.  Since the "Copyright" field
is mandatory, I've provided it but left it empty (with an explanatory
comment).