libdnssec: finish ED448 support

gnutls 3.6.12 adds support for this, given that you use sufficiently new
nettle (not released yet).  I tested this with Knot Resolver and
https://rootcanary.org/test.html

Note: in some switch statements I moved the labels inside #ifdef guards.
Otherwise the fall-through wouldn't be correct in the weird case of
not supporting ed25519 but supporting ed448 (shouldn't happen anyway).