debian/copyright: update syntax, maintainer, add license stanzas

Update the header paragraph to link to the canonical URL for the
format, and point to dev@ceph.io as the Contact.

Also add License: stanzas to reflect the licences in use (and refer to
fuller versions in /usr/share/common-licenses/ as appropriate).

This means that packages containing this copyright file are better in
compliance with the licences concerned.

Signed-off-by: Matthew Vernon <mvernon@wikimedia.org>