Update doxygen (a bit)

This is not a full update, but just updates some data channel-related docs
I came across. Other pages probably need a bit of attention too.

Stuff that was changed:
 * Explain data channel crypto format in crypto.h
 * Add P_DATA_V1 and P_DATA_V2 packet format spec
 * Remove '2.1' from title
 * Update some OpenSSL-specific text

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1417978095-19427-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9318
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b08c25dbaeffbdd80acc143a931a276163c851a3)