net/gnunet: update to 0.29, latest upstream

Release notes are at
https://www.gnunet.org/en/news/2026-08-0.29.0.html

Note that this is a major release, and incompatible with 0.28.
In FreeBSD-land, I don't know of any consumers outside of
Taler, so I'm not bumping anything else.

Some (minor) FreeBSD patches were submitted and accepted upstream.