Change openvpn_encrypt() to append to work buffer only

Change openvpn_encrypt() to append to work buffer only

Preparation for AEAD cipher modes, which also have to authenticate the
opcode and peer-id of packets.  To supply that information to
openvpn_encrypt(), I want to simply write those to the work buffer
before calling openvpn_encrypt().  That however requires that
openvpn_encrypt() never prepends something to the work buffer.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1454874438-5081-7-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11074
Signed-off-by: Gert Doering <gert@greenie.muc.de>