Rename mbedtls_net_close() to mbedtls_net_free()

close() may be more meaningful, but free() is symmetric with _init(), and more
consistent with all other modules
diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 9962445..7360d8a 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -615,9 +615,9 @@
     }
 #endif
 
-    mbedtls_net_close( &client_fd );
-    mbedtls_net_close( &server_fd );
-    mbedtls_net_close( &listen_fd );
+    mbedtls_net_free( &client_fd );
+    mbedtls_net_free( &server_fd );
+    mbedtls_net_free( &listen_fd );
 
 #if defined(_WIN32)
     mbedtls_printf( "  Press Enter to exit this program.\n" );