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/pkey/dh_client.c b/programs/pkey/dh_client.c
index 3ccfa6d..66fc471 100644
--- a/programs/pkey/dh_client.c
+++ b/programs/pkey/dh_client.c
@@ -281,7 +281,7 @@
 
 exit:
 
-    mbedtls_net_close( &server_fd );
+    mbedtls_net_free( &server_fd );
 
     mbedtls_aes_free( &aes );
     mbedtls_rsa_free( &rsa );