new option -s to override spoofing IP

Until now, the original sender's source IP was always reused for
outgoing packets. Recipients sometimes don't know a route to that
IP (remote LAN), but they know how to reach the sender's VPN IP.

So this option allows to cover the following scenario:

host1 and host2 are connected via an OpenVPN server.
Each host has its own LAN (lan1, lan2) and they do not have static
routes to the remote LAN.
The game software on host1 sends its broadcasts to lan1 instead to
the VPN interface. A router in lan1 uses udp-broadcast-relay and his
own OpenVPN tunnel to the same OpenVPN server to forward
broadcasts into the VPN. These are received on host2, but hosts2's
replies never make it to host1, as there is no routing for the LAN
addresses.
Using -s with the VPN IP of host1, the replies of host2 will be
directed to that same VPN IP of host1 and therefore reach their
destination.