keyfile: read MAC addresses and a test framework

Clean up handling of "special" keys in keyfiles, ie ones that
need more processing than the basic GKeyFile API supports.  Add
MAC address reading (writing support to come).

Additionally, add some test bits for the keyfile plugin that get
run on 'make check'.

system-settings/plugins/keyfile/plugin.c

@@ -350,7 +350,7 @@ add_connection (NMSystemConfigInterface *config,
                 NMConnection *connection,
                 GError **error)
 {
-	return write_connection (connection, NULL, error);
+	return write_connection (connection, KEYFILE_DIR, 0, 0, NULL, error);
 }
 
 static GSList *