Support for Flex 6K SDR radios, plus some fixes for easycomm

Two patches:

The first adds support for the Flex 6K series of SDR radios. These radios have a CAT application that runs on a windows machine, and exposes a network socket that can be opened for CAT commands. This back end connects to that network socket and provides an interface. I've tested pretty thoroughly using a Flex 6700 but I don't have the other models available (6500 and 6700R). CAT support on these models is evolving with frequent updates from Flex, and this back end does not reflect the latest features to be added. I hope to get to them soon.

The second patch makes the easycomm interface more robust by flushing the serial port and clearing the buffer.