vpnc-script-win: make VPN addresses/gateways "non-persistent", and delete them on...

vpnc-script-win: make VPN addresses/gateways "non-persistent", and delete them on disconnect

Persisting the state of IPv4 configuration of these interfaces is generally
unhelpful (as with DNS/WINS settings).

Two approaches. We do both:

1. Make the addresses ephemeral ("netsh ... store=active", as opposed to the
   "store=persistent" default). This ensures that the addresses won't last
   past reboot.
2. Delete the addresses upon disconnection.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>