Fix --tls-version-min and --tls-version-max for OpenSSL 1.1+

Fix --tls-version-min and --tls-version-max for OpenSSL 1.1+

As described in <80e6b449-c536-dc87-7215-3693872bce5a@birkenwald.de> on
the openvpn-devel mailing list, --tls-version-min no longer works with
OpenSSL 1.1.  Kurt Roeckx posted in a debian bug report:

"This is marked as important because if you switch to openssl 1.1.0
the defaults minimum version in Debian is currently TLS 1.2 and
you can't override it with the options that you're currently using
(and are deprecated)."

This patch is loosely based on the original patch by Kurt, but solves the
issue by adding functions to openssl-compat.h, like we also did for all
other openssl 1.1. breakage.  This results in not having to add more ifdefs
in ssl_openssl.c and thus cleaner code.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20180119212721.25177-1-steffan@karger.me>
URL: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=873302
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16284.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>