Treat dhcp-option DNS6 and DNS identical

Treat dhcp-option DNS6 and DNS identical

OpenVPN3 accepts both IPv4 and IPv6 with option-dhcp DNS but throws
an error for option-dhcp DNS6.

This patch makes OpenVPN2 accept IPv4/IPv6 for both DNS and DNS6

V2: Put IPv6 parsing logic into own function similar as for for IPv4 DNS
V3: more documentation / help message adjustments

Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <1517391662-21325-1-git-send-email-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16413.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>