linger time = 0 -> unconditional hard disconnect

[bmcdonnell]

Provide a way for an application using the Socket API to hard close a TCP socket when it wants to abort, regardless of whether there's any pending or unacked data.

Example application code:

void hardDisconnect(int sock)
{
   struct linger linger =
   {
     .l_onoff  = 1,
     .l_linger = 0,
   };

   lwip_setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof linger);
   lwip_shutdown(sock, SHUT_RDWR);
   lwip_close(sock);
}

References supporting this approach are included in the commit message.

Background discussion on bug #62231.

[bmcdonnell]

This worked well for me in bench testing with my own application (an embedded http client). I did not run the lwIP unit tests with it.

I think this patch (w/ appropriate application code like my example) may resolve the original issue in bug #62231, but the original poster reports he is unable to test further.