Port Windows code to NPTH.
* agent/gpg-agent.c (get_agent_ssh_socket_name): Use INVALID_HANDLE_VALUE instead of 0. (handle_signal) [!HAVE_W32_SYSTEM]: Don't define. (handle_connections): Port Windows code to NPTH. * dirmngr/dirmngr.c (handle_connections): Port Windows code to NPTH. * g13/g13.c (handle_connections): Port Windows code to NPTH. * scd/scdaemon.c (handle_connections): Port Windows code to NPTH.