Add `server.c`
This file contains connection-related function, client management, and other
stuff like this.
Add `loop()` to `server.c`
This function loops forever, waiting for connecting clients.
Rename `handle()` as `reply()`
The `handle` semantic will be used in `server.c` later.
Add `run()` to `server.c`
This function create a new thread to handle a client.
Change `run()` definition
Now, run is the new thread itself.
Bring back the old definition of `run()`
I changed my mind.

Actually, this is to be sure that the new thread will get one unique copy
of the client's file descriptor, by duplicating it with `malloc()`.
Add `handle()` to `server.c`
This function is called in a new thread each time a client joins in.
Add missing prototype of `start()` in `thd.h`
Proofreading
Some minor fixes:
	- Add a default value to `root`, I don't care of compiler warnings
	- Add a `len` variable in order to NUL-terminate the buffer in
	  `start()`, otherwise, calls to `split()` could seek forever in
	  the memory
	- Affectation directly in the declaration in `handle()`, just
	  to be more aesthetic
	- Add a missing newline
Add `main.c`
This file contains the `main()` function.

Options are parsed here.