DTLS/SCTP struct authchunks Bug

PR: 2809

DTLS/SCTP requires DATA and FORWARD-TSN chunks to be protected with
SCTP-AUTH.  It is checked if this has been activated successfully for
the local and remote peer. Due to a bug, however, the
gauth_number_of_chunks field of the authchunks struct is missing on
FreeBSD, and was therefore not considered in the OpenSSL implementation.
This patch sets the corresponding pointer for the check correctly
whether or not this bug is present.
(cherry picked from commit f596e3c491035fe80db5fc0c3ff6b647662b0003)
(cherry picked from commit b8140811367f6e1ef13afa6ffe9625309c46946c)
DTLS/SCTP Finished Auth Bug

PR: 2808

With DTLS/SCTP the SCTP extension SCTP-AUTH is used to protect DATA and
FORWARD-TSN chunks. The key for this extension is derived from the
master secret and changed with the next ChangeCipherSpec, whenever a new
key has been negotiated. The following Finished then already uses the
new key.  Unfortunately, the ChangeCipherSpec and Finished are part of
the same flight as the ClientKeyExchange, which is necessary for the
computation of the new secret. Hence, these messages are sent
immediately following each other, leaving the server very little time to
compute the new secret and pass it to SCTP before the finished arrives.
So the Finished is likely to be discarded by SCTP and a retransmission
becomes necessary. To prevent this issue, the Finished of the client is
still sent with the old key.
(cherry picked from commit 9fb523adce6fd6015b68da2ca8e4ac4900ac2be2)
(cherry picked from commit b9ef52b07897f249a9fa44943dba33fba8fb2721)
PBKDF2 should be efficient. Contributed by Christian Heimes
<christian@python.org>.