Change IPv4 routing setup on Windows to use CreateIpForwardEntry2()

[cron2]

As discussed in the context of commit 9c6d72c (https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25926.html) the current "install routes on Windows, with metric" is quite ugly, with the Vista+ workaround of using increasing metric values in a loop until things succeed.

The correct fix is to move this all to CreateIpForwardEntry2() and stop using the pre-Vista APIs.

This is something to tackle after 2.6 release, in the "early 2.7 cleanup / restructure" phase.