gnutls: security + maintenance update for both branches

Fixes #2813 CVE-2014-3466. No API/ABI changes in the updates.

Tests start to fail for gnutls31, so they get disabled,
but the 3.1 branch is already unused on master.

(cherry picked from commit 191dbc234fc3f9bfa7d265f0e9210c311cd7f6d6)