Deprecate key-method 1

Deprecate key-method 1

Key method 2 has been the default since OpenVPN 2.0, and is both more
functional and secure.  Also, key method 1 was only ever supported for
peer-to-peer connections (i.e. not for client-server).

Let's get rid of some legacy and phase out key method 1.

v2: add Changes.rst entry, and update man page

[ DS: Slightly modified patch, rewored the warning message and the
      Changes.rst note to encourage not to set --key-method at all ]

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1479153967-6788-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13054.html
Signed-off-by: David Sommerseth <davids@openvpn.net>