Put a prolog at the top of configuration files

Later I'm going to put in there some more information about the format etc.

src/common.c

@@ -1824,8 +1824,8 @@ read_config_file (struct str_map *config, struct error **e)
 }
 
 static char *
-write_default_config (const char *filename, const struct config_item *table,
-	struct error **e)
+write_default_config (const char *filename, const char *prolog,
+	const struct config_item *table, struct error **e)
 {
 	struct str path, base;
 
@@ -1861,6 +1861,9 @@ write_default_config (const char *filename, const struct config_item *table,
 		goto error;
 	}
 
+	if (prolog)
+		fputs (prolog, fp);
+
 	errno = 0;
 	for (; table->key != NULL; table++)
 	{