Add global header `thd.h`
This file will contain all function prototypes used elsewhere in the sources,
as well as common headers such as `<stdio.h>` or `<err.h>`.
Add `fs.h`
This header declares the global variable `root`, which represents the root
directory of the resources.

The default directory is "./": it should be modified on startup.
Add missing newline at the end of `thd.h`
Add `fs.c`
This file contains file-related functions.
Rename `path()` to `fullpath()`
Avoid namespace issues, especially when I want to call my variable `path`.
Add description to `exists()`
Add `PATH_MAX`
This constant specifies the maximum size of a path.

This will be used for both real paths and URI.
A real path may contain at most twice `PATH_MAX` characters (the root
directory's path, and the URI concatenated).

In fact, its value is *only* 256 while it should be 2048, according to the
standard. The reason is we don't really need so many characters to access
a resource.
Add `fullpath()` to `fs.c`
This function make a usable file path from a given URI.
Precisely, it concatenates the root directory's path with the URI.

If the result is too long (more than twice `PATH_MAX`), it returns NULL.

The result must be freed later with `free()`.