slirp: Cleanup resources on instance removal

Close & free sockets when shutting down a slirp instance, also release
all buffers.

CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
diff --git a/slirp/udp.h b/slirp/udp.h
index 9b5c3cf..9bf31fe 100644
--- a/slirp/udp.h
+++ b/slirp/udp.h
@@ -74,6 +74,7 @@
 struct mbuf;
 
 void udp_init(Slirp *);
+void udp_cleanup(Slirp *);
 void udp_input(register struct mbuf *, int);
 int udp_output(struct socket *, struct mbuf *, struct sockaddr_in *);
 int udp_attach(struct socket *);