Add `compare()` to `auth.c`
Directly give `strcmp()` to `qsort()` was wrong.
Change it with `compare()`.
Add `fs.h` to `thd.h`
The functions prototyped in `fs.h` are required in all the sources.
Use `fopen()` instead of `open()`
This will be simpler to parse each line with `fgets()` next.