Update: Add missing function f_path_is_absolute() and fix existing f_path_is_*()...

Update: Add missing function f_path_is_absolute() and fix existing f_path_is_*() functions.

The f_path_is_absolute() function, being the compliment of f_path_is_relative() is now added.

I noticed multiple problems when looking at this code.
- The f_path_is_relative() and f_path_is_relative_current() functions are not checking that the max length is reached before comparing.
- the f_path_is_relative_current() is not incrementing the counter when attempting to check for the next character resulting in invalid results.

The f_path project clearly needs unit testing.
I intend to write unit tests and fix problems found before the next stable release is made.