Don't print `Ignoring unknown extended header` message for each file

[Pavel Raiskup]

Per report [1], do you think that we could hash the already reported
messages somewhere, and skip printing them repeatedly on stderr (for each
file), and _by default_?

  $ tar tf some.star.gz
  ...
  /usr/bin/tar: Ignoring unknown extended header keyword 'SCHILY.dev'
  /usr/bin/tar: Ignoring unknown extended header keyword 'SCHILY.ino'
  ...
  /usr/bin/tar: Ignoring unknown extended header keyword 'SCHILY.dev'
  /usr/bin/tar: Ignoring unknown extended header keyword 'SCHILY.ino'
  ...

Perhaps we could print
  ...
  /usr/bin/tar: Ignoring unknown extended header keyword 'SCHILY.dev'
  ...
  /usr/bin/tar: The --no-repeated-errors filter: Ignoring unknown extended 
header keyword 'SCHILY.dev'.
  ...

Of course we could add support for those headers...  but that is different
thing now.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1823866

Pavel