journalctl-filter: several cleanups for add_matches()

journalctl-filter: several cleanups for add_matches()

- split it into small pieces to reduce indentation,
- return earlier if no filter specified,
- use journal_add_matchf() and journal_add_match_pair(),
  chase_and_stat(), is_symlink(), and sd_device_new_from_devname(),
- replace sd_device_get_devname() + stat() with sd_device_get_devnum(),
- refuse to install filter by path with --root, --image, or --machine
  at least now, as previously it has not worked anyway.