Feature Request: make `multihome` on-by-default

[cron2]

https://github.com/OpenVPN/openvpn/issues/304 suggests that on modern installations, --multihome is needed more often nowadays, with systems having IPv6 privacy addresses (= rotating addresses).

This affects mostly --server setups, and p2p setups on the listening side.

TODO:

• test multihome on all platforms
• decide how to proceed (rip out non-multihome code, or flip the default and introduce --no-multihome to use the old code path if the new one is not working right)
• submit code :-)