Allow certain DHCP options to be used without DHCP server

Allow certain DHCP options to be used without DHCP server

Followin DHCP options:

  DOMAIN, ADAPTER_DOMAIN_SUFFIX, DNS, WINS

don't require DHCP server in order to be used.

This change allows those options to be used with dco and wintun
drivers. If an option specified which requires DHCP server and
tap-windows6 driver is not used, print a clear error message
instead of obscure reference to --ip-win32.

Reported-by: Marek Zarychta
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20230207145416.1415-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26169.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>