not reusing the "options" name

There were multiple different objects named by "options". Now there is
only one object named "options", which is the nearby file. The other
objects have been renamed to ctypesgen_options.
remove trailing newlines from python files
spruce up options.py docstring
fix typo in load_ctypesgen_options docstring
make is_win more testable

Pass in the platform as a parameter, rather than using sys from global
scope. This avoids using a module like mock when testing the is_win
method.
write a simple test for is_win