Refactor and move the block-outside-dns code to a new file (block_dns.[ch])

Refactor and move the block-outside-dns code to a new file (block_dns.[ch])

- Move the core of win_wfp_block_dns() to a new function
- Remove globals and make it independent of the rest of the code

This facilitates implementing support for block-outside-dns through
the interactive service. Should not change any functionality.

v2 changes:
- In comments, correct DeleteBlockDNS() to delete_block_dns_filters

v2a: added <winsock2.h> and <ws2ipdef.h> (Gert Doering)

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1456457091-3872-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11264
Signed-off-by: Gert Doering <gert@greenie.muc.de>