Set a low interface metric for tap adapter when block-outside-dns is in use

Set a low interface metric for tap adapter when block-outside-dns is in use

Windows 10 before Creators Update used to resolve DNS using all
available adapters and IP addresses in parallel. Now it still
resolves addresses using all available adapters but in a round-robin
way, beginning with random adapter.
This behaviour introduces significant delay when block-outside-dns is
in use. Fortunately, setting low metric for the TAP interface solves
this issue, making Windows always pick TAP adapter first and disable
round-robin.

Signed-off-by: ValdikSS <iam@valdikss.org.ru>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20170510184753.27145-1-valdikss@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14624.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 27aa87283f6e766507287649aa5a63f1f5172645)