Move openvpn specific key expansion into its own function

Move openvpn specific key expansion into its own function

This moves the OpenVPN specific PRF into its own function also
simplifies the code a bit by passing tls_session directly instead of
5 of its fields.

Patch v2: Rebase

Patch v4: rewrite/fix comments,
          fix potential not initialised before goto issue

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20200825073643.15920-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20815.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>