Too many newlines at the end of the file
Add `allowed()` to `auth.c`
This function checks if a given resource is authorized in the permission file
previously loaded with `permissions()` *and* have read permissions in the
filesystem.
Add prototypes in `thd.h`
Functions defined in `auth.c` are now prototyped in `thd.h`.
Oops, bring back `root` in `fs.c`
The linker doesn't seem to like the disappearance of `root`: declare it at
the top of the source instead.

The definition of `root` *must* be done before using functions contained
in `fs.c`.