Add options to restrict cipher negotiation

Add options to restrict cipher negotiation

Add --ncp-disable to completely disable cipher negotiation, and
--ncp-ciphers to specify which ciphers to accept from the server.

v2:
 * fix --disable-crypto builds
 * use register_signal() instead of operating directly on c->sig
 * add man-page entry for new options

v3:
 * rebased on client-side NCP v3

v4:
 * rebased on client-side NCP v4

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1467149700-10042-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/12008
Signed-off-by: Gert Doering <gert@greenie.muc.de>