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/ssl/ssl_client1.c b/programs/ssl/ssl_client1.c
index 61ead50..4b99a15 100644
--- a/programs/ssl/ssl_client1.c
+++ b/programs/ssl/ssl_client1.c
@@ -290,7 +290,7 @@
     }
 #endif
 
-    mbedtls_net_close( &server_fd );
+    mbedtls_net_free( &server_fd );
 
     mbedtls_x509_crt_free( &cacert );
     mbedtls_ssl_free( &ssl );