On windows, when the why3server has accepted a client on the server
socket, it continues to use that socket for the client communication. A
new socket is created so that new clients can connect. However, there
may be a little time between accepting the client and creating the new
socket, where the connection of another client will be refused. This
resulted in crashes.

We now simply prepare more than one server socket, so that several
clients can connect at the same time. More precisely, we create as many
sockets as gnatwhy3 processes will be run in parallel.

Concretely, we replace the previous server_socket and server_key
variables by arrays, whose length is determined by the "parallel"
option. The code which manipulated these variables needs to be adapted
to know which of the sockets to manipulate.

* server-win.c
(shutdown_with_msg): close all handles
(create_server_socket): new integer argument which specifies the cell to
   store the new server socket in
(accept_client): new integer argument which specifies on which of the
   server sockets the connection was accepted. Used to create a new
   socket in that cell using create_server_socket.
(init): create an array of sockets instead of just one
(get_server_num): new function to determine the socket which got the
  new client
(main): use get_server_num to know if it was a server socket that had an
   event, and which one. Call accept_client with the result.

Change-Id: I5f7ff1b30c9340d897b75085ade61b2d79f87167
(cherry picked from commit f2cbded5da7a3fcbebb61ee98b8e6c8d9c1d2227)