MAVProxy does not work with IPv6

MAVProxy does not work when the host is specified as an IPv6 address.

When trying to open MAVProxy with IPv6, argument parsing is broken:

e.g. `sim_vehicle.py "--out=udp:[::]:14550"`
> "UDP ports must be specified as host:port"

Also the socket connection code uses `socket.AF_INET` instead of `socket.AF_INET6` so it will not use IPv6 even if the parsing is corrected.

Start at the argument-parsing path exercised by sim_vehicle.py with `--out=udp:[::]:14550`, then trace the socket connection code mentioned in the issue. Reproduce the current port-parsing error and IPv4-only behavior; done means the IPv6 address parses and the socket connection uses IPv6 for this case.

Written by the indexing model from the issue text.