This is a revision of the test_intermixed.py that I submitted earlier. Now `parse_intermixed_args` acts like `parse_args', and calls `parse_known_intermixed_args`. Again it is form that can exercise the idea without modifying `argparse.py`.
If the parser has incompatible features (REMAINDER, PARSER, or certain exclusive groups), it raises an error. However to facilitate testing I included a `_fallback` backdoor. If not default None it will be called instead of raising the error.
While making documentation changes, I got to wondering whether 'interspersed' would be a better term than 'intermixed'. optparse has an 'interspersed' option and api. However the getopt documentation does use 'intermixed'.