Add --route-pre-down/OPENVPN_PLUGIN_ROUTE_PREDOWN script/plug-in hook

Add --route-pre-down/OPENVPN_PLUGIN_ROUTE_PREDOWN script/plug-in hook

This patchs adds a script/plug-in hook which is called right before the
network routes are taken down.  This gives external processes a
possibility to tear down communication over the VPN before the VPN
disappears.

One use case can be to mount a networked file system over the VPN via
--route-up.  And then to unmount this file system via --route-pre-down

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>