Add `http.c` and `http.h`
Those files provide functions to handle all HTTP-related stuff.
Add `split()` to `http.c`...
...and some other things.

Actually, there is no word to describe the mess I am trying to develop.
Add `get()` to `http.c`
This function handles GET requests.
Add `header()` to `http.c`
This function sends the corresponding status line for a given URI.
Oops, I have forgotten little things
Prototype of `header()` was missing.
A semicolon (damn, semicolons) was missing.
Add `transmit()` to `http.c`
This function sends to a file descriptor the content of another file
descriptor.

Because we're dealing with HTTP, a "Content-Length" field is sent first.
Missing parenthesises
Add `handle()` prototype in `thd.h`