Warn that DH config option is only meaningful in a tls-server context

Warn that DH config option is only meaningful in a tls-server context

If specified in a tls-client context, don't try to open the file as it's
not used. Worse even, if 'none' was specified to disable explicitly, it
complained that the file 'none' could not be found.

[DS: On-the-fly update - Prefixed the message with 'WARNING: ']

Signed-off-by: Gert van Dijk <gert@gertvandijk.net>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170827161515.2424-1-gert@gertvandijk.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15332.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 47a0a80b7718fe88451c82bdfe838e5a6e3c4248)