DTLS handshake fix.

Reported by: Prashant Jaikumar <rmstar@gmail.com>

Fix handling of application data received before a handshake.
(cherry picked from commit 0c75eeacd3285b395dc75b65c3e6fe6ffbef59f0)
DTLS message_sequence number wrong in rehandshake ServerHello

This fix ensures that
* A HelloRequest is retransmitted if not responded by a ClientHello
* The HelloRequest "consumes" the sequence number 0. The subsequent
ServerHello uses the sequence number 1.
* The client also expects the sequence number of the ServerHello to
be 1 if a HelloRequest was received earlier.
This patch fixes the RFC violation.

Conflicts:

	ssl/d1_pkt.c
(cherry picked from commit 6f87807e629ee10ec0006b39d8851af8c5ade67b)