Change freedb references to gnudb

[MilesBHuff]

freedb was shut down. This PR to johnlane's fork updates the URLs in abcde to use gnudb. It might be a good idea to incorporate it?

[poddmo]

Good idea. A quick search of abcde found 13 occurrences of freedb. Do you want to submit a pull request here or leave it with me?

[MilesBHuff]

I would prefer to leave it to someone more-familiar with the project. I could of course just follow what was done in the above PR, but I know your fork differs from the last mainline abcde a fair bit, including in regards to another upstream source, MusicBrainz. So if something more than the above PR needs doing, you'd be far-better-equiped than I to identify and perform it.