Add support for Flex6K radios

Signed-off-by: Steve Conklin <steve@conklinhouse.com>
Add paramater checking, serial port flush, buffer clear, and additional debug output

Signed-off-by: Steve Conklin <steve@conklinhouse.com>
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.

Steve, AI4QR

Merge branch 'for-upstream' of https://github.com/sconklin/hamlib into sconklin-for-upstream