Drop recursively routed packets

Drop recursively routed packets

v2: better method naming

On certain OSes (Windows, OS X) when network adapter is
disabled (ethernet cable pulled off, Wi-Fi hardware switch disabled),
operating system starts to use tun as an external interface.
Outgoing packets are routed to tun, UDP encapsulated, given to
routing table and sent to.. tun.

As a consequence, system starts talking to itself on full power,
traffic counters skyrocket and user is not happy.

To prevent that, drop packets which have gateway IP as
destination address.

Tested on Win7/10, OS X.

Signed-off-by: Lev Stipakov <lstipakov@gmail.com>
Trac: 642
Tested-by: ValdikSS <iam@valdikss.org.ru>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1451911424-12970-1-git-send-email-lstipakov@gmail.com>
URL: https://sourceforge.net/p/openvpn/mailman/message/34737757/
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit e9d64bc03742c96a3d7fe2a473c43d40e5ba2001)