Regression: Fix the preserve and offset calculation in Featureless Make.

Regression: Fix the preserve and offset calculation in Featureless Make.

This addresses the problems observed in commit 3befe37125e426d452a1b9b9a1fab97ab221c7f1.
I never got the time to investigate this before I made my end of day commit.
This now addresses the observed problems.

The offset application logic is accidentally reversed.
This is now corrected.

The new logic does not require the buffer because the directory is being created later on in the process.
Using the `f_string_append_nulless()` to easily copy the source path with the offset onto the destination path.

Update the documentation comments to better communicate this behavior.