Add support for version number without trailing ".0"

* packages/gen-frameworks.sh: Handle version number with and without
trailing ".0" and report invalid version numbers. Add a few variables
to reduce duplication of hardcoded URLs and filenames.

So far all frameworks releases end in ".0". I ran the script without
trailing ".0" and it took some time to realize why it didn't work.

• GnuPG-bug-id: T5592