Update: directory and path changes, other fixes and changes.

Update: directory and path changes, other fixes and changes.

f_directory and f_path overlap.
I believe that f_path is the better choice for getting the current directory/path and related.

Add f_path_change(), f_path_change_at(), f_path_current(), and f_path_real().

Add F_absolute and F_relative status codes.

The define FL_status_string_recurse_length did not include the leading f_ in the derived length.

Minor documentation comment changes.