config file: don't print warning if config file cannot be opened

dist/ifupdown-ng.conf.example

@@ -0,0 +1,10 @@
+# This is an example configuration file for ifupdown-ng, which allows
+# the system administrator to configure the behaviour of ifupdown-ng.
+#
+# The settings specified here are the defaults of ifupdown-ng.
+
+# allow_addon_scripts:
+# Enable support for /etc/if-X.d addon scripts.  These are used for
+# compatibility with legacy setups, and may be disabled for performance
+# improvements in setups where only ifupdown-ng executors are used.
+allow_addon_scripts = 1

libifupdown/config-file.c

@@ -52,8 +52,10 @@ lif_config_load(const char *filename)
 
 	if (fd == NULL)
 	{
+#if 0
 		fprintf(stderr, "ifupdown-ng: cannot open config %s: %s\n",
 			filename, strerror(errno));
+#endif
 		return false;
 	}