Simplify net_accept() with UDP sockets

This is made possible by the new API where net_accept() gets a pointer to
bind_ctx, so it can update it.
diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 7360d8a..76e3b31 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -533,6 +533,8 @@
      * 2. Wait until a client connects
      */
 accept:
+    mbedtls_net_free( &client_fd );
+
     mbedtls_printf( "  . Waiting for a remote connection ..." );
     fflush( stdout );
 
@@ -544,20 +546,6 @@
     }
 
     mbedtls_printf( " ok\n" );
-    fflush( stdout );
-
-    mbedtls_printf( "  . Re-bind on UDP/%s/%s ...",
-            opt.listen_addr, opt.listen_port );
-    fflush( stdout );
-
-    if( ( ret = mbedtls_net_bind( &listen_fd, opt.listen_addr, opt.listen_port,
-                          MBEDTLS_NET_PROTO_UDP ) ) != 0 )
-    {
-        mbedtls_printf( " failed\n  ! mbedtls_net_bind returned %d\n\n", ret );
-        goto exit;
-    }
-
-    mbedtls_printf( " ok\n" );
 
     /*
      * 3. Forward packets forever (kill the process to terminate it)