mavutil: Add support for udpin and udpout style addresses.

Adds two new address forms which can be used in mavproxy --master, --out
etc...

* udpin:addr:port - Run as a UDP 'server' on the specified interface
(use 0.0.0.0 for any interface) and port.
* udpout:addr:port - A UDP client that connects to a particular server

# Example usage

On a coprocessor linux computer connected directly to a APM:
mavproxy.py --master=/dev/ttyACM0,115200 --out=udpin:0.0.0.0:14550

On a laptop PC connecting wirelessly to that coprocessor:
mavproxy.py --master=udpout:<coprocessorip>:14550