Update: Attempt to avoid libc/POSIX character array length limitations.

Update: Attempt to avoid libc/POSIX character array length limitations.

Define f_string_t_length to handle the special case max length.
Make the f_utf_string_t also operate in the same way (even it it might avoid the problem by being a different type than char).
The f_utf_string_t is changed in this way for consistency reasons.