Update: Change fl_directory_do() to make the action optional as well.

Update: Change fl_directory_do() to make the action optional as well.

The action flag for `fl_directory_do()` should also be optional.
Adjust the code so that recursion still happens even if the action flag is not set.

This better allows for cases where the before is to be used but the action itself is not needed.
This happens to be the case with Featureless Make.

Add additional helper flags.

Improve documentation comments.
Add a warning about setting the top string to a string that is changed during recursion.