devel/gnupg25: new port 2.5.17, latest upstream

2.5.16 was (kind of suddenly, since I was waiting for 2.6) declared the
stable version. This was probably because of gpg.fail (which was partly
addressed by the previous update to 2.4.9).

Then 2.5.17 was released to address a number of critical security issues
introduced in the 2.5 branch. Release notes and discussion can
be found at
https://lwn.net/Articles/1056209/
Upstream release notes at
https://dev.gnupg.org/T7996

Marked as conflicting with gnupg1 and gnupg-2.4* because the names
of executables have changed again, and the "is-gpg2" flags went
away, so co-installability is poor.