Update: Improve the logic for the read link f_file functions.

Update: Improve the logic for the read link f_file functions.

The f_file_link_read() and f_file_link_read_at() functions have their dynamic allocation logic improved.
Use link_stat.st_size rather than target->used to pass to readlink() or readlinkat().

Initialize target->used to 0 and then update target->used on success while ensuring the string is always NULL terminated after the target->used.

The unit tests revealed that these can and should be improved.