initial work for issue 399: add --force flag

See #399.
Use True/False/None for boolean parameters instead of 0 or 1.

@jessicamizzi using default parameters is a good way to avoid breaking code that is already using check_file_status,
but since all uses currently are internal (they doesn't change the public API) tracking all occurrences in the codebase and refactoring is still feasible.