all: Support syncing extended attributes (fixes #2698) (#8513)

This adds support for syncing extended attributes on supported
filesystem on Linux, macOS, FreeBSD and NetBSD. Windows is currently
excluded because the APIs seem onerous and annoying and frankly the uses
cases seem few and far between. On Unixes this also covers ACLs as those
are stored as extended attributes.

Similar to ownership syncing this will optional & opt-in, which two
settings controlling the main behavior: one to "sync" xattrs (read &
write) and another one to "scan" xattrs (only read them so other devices
can "sync" them, but not apply any locally).

Co-authored-by: Tomasz Wilczyński <twilczynski@naver.com>

lib/fs/basicfs_fileinfo_linuxish.go

@@ -0,0 +1,22 @@
+// Copyright (C) 2022 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+//go:build aix || dragonfly || linux || openbsd || solaris || illumos
+// +build aix dragonfly linux openbsd solaris illumos
+
+package fs
+
+import (
+	"syscall"
+	"time"
+)
+
+func (fi basicFileInfo) InodeChangeTime() time.Time {
+	if sys, ok := fi.FileInfo.Sys().(*syscall.Stat_t); ok {
+		return time.Unix(0, sys.Ctim.Nano())
+	}
+	return time.Time{}
+}