Support --block-outside-dns on multiple tunnels

Support --block-outside-dns on multiple tunnels

v2: Simplify the "add sublayer" code.

Currently each instance of openvpn adds WFP filters into an independent
sublayer. As a block in one sublayer can over-ride a permit in another,
this causes all DNS traffic to block when --block-outside-dns is used
in multiple tunnels.

Fix using a common sublayer for adding firewall rules (filters) from all
instances of openvpn and interactive service.
- The sublayer is added in a persistent session so that it could be
  accessed from multiple sessions.
- The sublayer is identified by a fixed UUID defined in block_dns.c
  shared between openvpn.exe and openvpnserv.exe.
- Permit filters for tun/tap interfaces are added with higher priority
  than filters that block all DNS traffic. This is not strictly
  necessary as WFP assigns higher priority to specific filters over generic
  ones, but it may be safer not to rely on that feature.
- All filters are added in dynamic sessions as before. They get
  automatically removed when the process exits. The sublayer will,
  however, persist until reboot.

Resolves Trac 718
Tested on Windows 7, 10 with/without interactive service

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1474085439-28766-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12465.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>