packages: Rename unbound to local-unbound

This more accurately reflects its purpose, and its contents, since
everything in the package is prefixed with "local-".

While here, add a message on upgrade about regenerating the config.

MFC after: 3 seconds
Requested by: des
Reviewed by: des
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53056

(cherry picked from commit bf84861460f9287fe9cf66b2cd22fefe8e242a28)