Poor man's NCP for non-NCP peers

Poor man's NCP for non-NCP peers

Allows non-NCP peers (<= 2.3, or 2.4+ with --ncp-disable) to specify a
--cipher that is different from the one in our config, as long as the new
cipher value is allowed (i.e. in --ncp-ciphers at our side).

This works both client-to-server and server-to-client.  I.e. a 2.4 client
with "cipher BF-CBC" and "ncp-ciphers AES-256-GCM:AES-256-CBC" can connect
to both a 2.3 server with "cipher BF-CBC" as well as a server with
"cipher AES-256-CBC" in its config.  The other way around, a 2.3 client
with either "cipher BF-CBC" or "cipher AES-256-CBC" can connect to a 2.4
server with e.g. "cipher BF-CBC" and "ncp-ciphers AES-256-GCM:AES-256-CBC"
in its config.

This patch was inspired by Gert's "Poor man's NCP for 2.3 clients" patch,
but takes a different approach to avoid the need for server-side scripts
or client-side 'setenv UV_*' tricks.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1479936104-4045-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13218.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>