fs/io/BufferedReader: new class to replace class TextFile

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -168,12 +168,9 @@ SimpleDatabase::Load(Error &error)
 	assert(!path.IsNull());
 	assert(root != nullptr);
 
-	TextFile file(path);
-	if (file.HasFailed()) {
-		error.FormatErrno("Failed to open database file \"%s\"",
-				  path_utf8.c_str());
+	TextFile file(path, error);
+	if (file.HasFailed())
 		return false;
-	}
 
 	if (!db_load_internal(file, *root, error))
 		return false;