Refactor data channel key generation API

Refactor data channel key generation API

Originally for "poor man's NCP", I introduced a simpler API for generating
data channel keys.  That refactoring is no longer needed for that patch,
but I believe still worth a patch on it's own.

This patch should not change any functionality.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1479931325-25919-2-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13216.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>