Make "block-outside-dns" option platform agnostic

Make "block-outside-dns" option platform agnostic

Make the "block-outside-dns" option agnostic of Windows versions by
dynamically loading WFP-related functions. Cross-compiled on Linux and
tested on Windows XP/10.

v2: move MinGW definitions to win32_wfp.h and add attribution.

v3: keep #ifdef WIN32 in init.c (do not break non-windows platforms).

v4: Also make MSVC happy.

Tested-by: ValdikSS <iam@valdikss.org.ru>
Tested-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1450125695-36596-1-git-send-email-fish.thss@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10795

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