Added support for DHCP option 119 (dns search suffix list) for Windows.

Added support for DHCP option 119 (dns search suffix list) for Windows.

As of Windows 10 1809 Windows finally supports this so it makes sense
to add support to OpenVPN as well.

Multiple options can be specified at the same time, with one search
domain per line (in the config, or pushed from server):

  dhcp-option DOMAIN-SEARCH my.company.domain
  dhcp-option DOMAIN-SEARCH some.example.domain

OpenVPN will (on windows) concatenate them all together into a single
"option 119" for the tapv9 DHCP server.  Max length is 254 in total.

DNS label compression is not used - it's complicated, and Windows does
not need it.  See RFC 3397 for more details.

This only works with the tun/tap driver, not with wintun.

On non-windows platforms, these settings are exported in the environment
towards --up scripts (or to the management interface), and need to be
picked up there.

Signed-off-by: Jan Just Keijser <jan.just.keijser@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <c404dd17-e0db-ce61-0d79-864a5736f2d0@nikhef.nl>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20349.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>