live-snapshot: fixed include-list handling.

live-snapshot: fixed include-list handling.

* When "/etc/live-snapshot.list" is present, all standard files and
  directories listed there are saved on shutdown.
  Now it saves them only if they are modified, and also handles
  dangling symbolic links. This beaviour will be gentle to live-systems
  upgrades on the same hardware since new unmodified to-be-snapshotted files
  will be no more overwritten by old unnecessary saved cruft.
  It still does not handle removal of deleted files.

Signed-off-by: Marco Amadori <amadorim@vdavda.com>