net: do not depend on slirp internals

Only slirp/libslirp.h should be included.

Instead of using some slirp declarations and utility functions directly,
let's copy them in net/util.h.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>