[regression] 2.12.23-12ubuntu2.4 breaks sha512 certificates

[Kees Cook]

$ gnutls-cli -p 587 smtp.outflux.net -s --print-cert
STARTTLS
ctrl-D
*** Starting TLS handshake
*** Fatal error: The signature algorithm is not supported.
*** Handshake has failed

This does not happen with 2.12.23-12ubuntu2.3.

$ echo QUIT | openssl s_client -connect smtp.outflux.net:587 -starttls smtp -showcerts 2>/dev/null | openssl x509 -noout -text
...
    Signature Algorithm: sha512WithRSAEncryption
...
            Public Key Algorithm: rsaEncryption
...

There's no MD5 visible.

[Kees Cook (kees)]

Thanks to mdeslaur for finding that the _root_ cert is the problem, not mine, nor a code problem with gnutls:

http://blog.cacert.org/2015/12/re-signing-root-certificate/