[RFC] Re-entrancy in http module

Hi,

I'm using the HTTP server module from gwlib in another application.

I stop and start the server module several times during operations using
http_init() and http_shutdown ().

I've had to make some patches to get this to work.

The function

gwthread_join_every(server_thread);

is invoked from

http_close_all_ports() and server_shutdown().

After this, I think there should be;

server_thread_is_running = 0;

so that the HTTP server "state machine" is reset completely.

Also the calls to 

fdset_destroy(server_fdset) and fdset_destroy(client_fdset)

should be followed with 

xxx_fdset = NULL

for the same reason.

I know that this does not affect the core operation of Kannel right now,
but it seems useful to have this code as reusable elsewhere.