Fix exclude_by_attribute feature on POSIX

The exclude by attribute function is broken on non-Darwin POSIX: linux and freebsd.
This is because those xattrs must be prefixed by a legal namespace. The old xattr
library implicitly appended the user namespace to the xattr, but the current
official go pkg does not (which is just as well).

Also fix the test to remove the discordant old xattr dependency and provide
test cases for both darwin and non-darwin POSIX.

src/duplicacy_utils_linux.go

@@ -1,13 +0,0 @@
-// Copyright (c) Acrosync LLC. All rights reserved.
-// Free for personal use and commercial trial
-// Commercial use requires per-user licenses available from https://duplicacy.com
-
-package duplicacy
-
-import (
-)
-
-func excludedByAttribute(attirbutes map[string][]byte) bool {
-	_, ok := attirbutes["duplicacy_exclude"]
-	return ok
-}