Add `line()` to `auth.c`
This function returns a pointer to the next line of a given file stream.
If the end of file is reached, returns NULL.
Add `fill()` to `auth.c`
This function tries to fill a given buffer with exactly one line of
a given file stream.

Some sign-related stuff has been fixed too.
Fix little things in `fs.c`
Add an newline, and remove the affectation of `root`, because of incompatible
pointer qualifiers.
Improve `fill()` in `auth.c`
Little optimization of `fill()`: the function calls only once `strchr()` to
get the newline character, check if the line has been truncated, and get
the length of the line.

Just in one call.