Progress: get the change file mode functions in an acceptable state.

Progress: get the change file mode functions in an acceptable state.

Finish writing the file change mode and related functions.
Due to significant gaps in development and the notable complexity of the ugo+rwxXst modes, this is implemented in what appears to be good enough for now.
There are likely hidden logic flaws or incomplete/missing logic.

This adds an @fixme to more thoroughly review this and possible fix this at a later date.
For now, this allows moving forward to avoid spending too much time on this one area.