Add MAVProxy as cygwin installer package

I'm making ardupilot/tools/scripts/install-prereqs-cygwin.sh to automatically install all the packages we need for Windows users. If MAVProxy was submitted to cygwin as a package then it can be pragmatically installed auto-magically.

See https://cygwin.com/setup.html for details

Start with Cygwin's setup documentation and inspect ArduPilot's tools/scripts/install-prereqs-cygwin.sh, which the issue identifies as the installer entry point. Determine the process for submitting MAVProxy as a Cygwin package and verify that the package can be installed automatically by the prerequisite script.

Written by the indexing model from the issue text.