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_mail_client.c b/programs/ssl/ssl_mail_client.c
index 4ea5d4c..27e7d5a 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -819,7 +819,7 @@
 
 exit:
 
-    mbedtls_net_close( &server_fd );
+    mbedtls_net_free( &server_fd );
     mbedtls_x509_crt_free( &clicert );
     mbedtls_x509_crt_free( &cacert );
     mbedtls_pk_free( &pkey );