Port fsync error handling from restic

This ignores several different combinations of errnos which are returned
if the storage destination is not able to fsync correctly.

See also https://github.com/restic/restic/pull/4021

repo/repo_unix.go

@@ -0,0 +1,19 @@
+//go:build !windows
+// +build !windows
+
+package repo
+
+import (
+	"errors"
+	"runtime"
+	"syscall"
+)
+
+// The ExFAT driver on some versions of macOS can return ENOTTY,
+// "inappropriate ioctl for device", for fsync.
+//
+// https://github.com/restic/restic/issues/4016
+// https://github.com/realm/realm-core/issues/5789
+func isMacENOTTY(err error) bool {
+	return runtime.GOOS == "darwin" && errors.Is(err, syscall.ENOTTY)
+}