add support for --dns option

add support for --dns option

As a first step towards DNS configuration in openvpn and a unified way
to push DNS related settings to clients in v2 and v3, this commit adds
support for parsing the new --dns option. Later commits will add support
for setting up DNS on different platforms.

For now, --dns and DNS related --dhcp-option can be used together for
smoother transition. Settings from --dns will override ones --dhcp-option
where applicable.

For detailed information about the option consult the documentation in
this commit.

Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220323143452.1100446-1-heiko@ist.eigentlich.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23997.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>