Bugfix: Fixes to f_path of problems exposed by unit tests.

Bugfix: Fixes to f_path of problems exposed by unit tests.

Remove unnecessary variable.
Remove unnecessary if condition.

Change while loop to be more robust in .
Check the path.used before checking path.string in case path.used is 0 (this should be safer).

Update function documentation.

The logic in private_f_path_real() is incorrect.
The realpath() function returns a string (char *) rather than an integer.

Also do some code clean up.