Function to get last error with minor VisualStudio improvements

[hgletthofer]

Libmodbus on Windows will build by default a shared library (DLL) that is statically linked against VCRedist from the used compiler. This prevents the application from accessing the errno variable.

I added a method 'modbus_get_error' that will allow you to access this value. In addition i updated the VisualStudio solution/project file to have a working 64-bit configuration as well as different build directories.

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