OpenVPN: Reorder ifconfig for N2N [Do not merge this]

OpenVPN: Reorder ifconfig for N2N [Do not merge this]

N2N currently reserves a /24 subnet for each connection. This patch should spare network space
cause the P2P topology only needs a /30 subnet.

Open Questions:
- The firewalling of N2N needs to be adapted (and deeper checked in general) since openvpnctrl uses always a DROP for *.0/30 for OVPNBLOCK.
- An overlaping of different N2N subnets should be prevented.
- More testings needs to be done also with already existing connections.

Signed-off-by: Erik Kapfer <ummeegge@ipfire.org>