archivers/libcabinet: add LICENSE and switch to new MASTER_SITES and WWW

While here, bump DISTVERSION to 0.31. No functional changes. The
version bump is only indicative of new maintenance and the ability of
others/users to more easily participate in it's development.

Summary of changes:

• New home: https://codeberg.org/BSDforge/libcabinet
• take maintainership
• add LICENSE file
• add README.md

PR: 287759
Reported by: Chris Hutchinson <portmaster@BSDforge.com> (new maintainer)