bug#4953: 23.1.50; Dired does not deal well with xz-compressed files

While finding xz-compressed files is handled transparently in Emacs, the
same cannot be said about their support (or rather, lack thereof) in
Dired.  Two examples:

- dired-do-compress on file.xz produces file.xz.gz instead of file

- dired-do-shell-command does not offer anything for file.tar.xz (with
  GNU tar 1.22, just "tar xf" unpacks it, older versions may need the
  option --use-compress-program=xz).


In GNU Emacs 23.1.50.3 (i486-pc-linux-gnu, GTK+ Version 2.18.3)
 of 2009-11-17 on turtle
Windowing system distributor `The X.Org Foundation', version 11.0.10605000
configured using `configure  '--build=i486-pc-linux-gnu' 'build_alias=i486-pc-linux-gnu''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: C
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: de_DE.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Dired by name

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-x k <return> C-x d <M-backspace> <M-backspace> <M-backspace> 
<return> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> Z y Z y g M-x r e p o r t - e 
m <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Compress or uncompress coreutils-8.0.128-630f.tar.xz? (y or n) 
Compressing /usr/local/src/coreutils-8.0.128-630f.tar.xz...done
Compress or uncompress: 1 file.
Compress or uncompress coreutils-8.0.128-630f.tar.xz.gz? (y or n) 
Uncompressing /usr/local/src/coreutils-8.0.128-630f.tar.xz.gz...done
Compress or uncompress: 1 file.

Load-path shadows:
None found.

Features:
(shadow mail-extr message ecomplete rfc822 mml easymenu mml-sec
password-cache mm-decode mm-bodies mm-encode mailcap mail-parse rfc2231
rfc2047 rfc2045 qp ietf-drums mailabbrev nnheader gnus-util netrc
time-date mm-util mail-prsvr gmm-utils wid-edit mailheader canlock sha1
hex-util hashcash mail-utils emacsbug sendmail dired-aux dired
regexp-opt tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win
x-dnd font-setting tool-bar dnd fontset image fringe lisp-mode register
page menu-bar rfn-eshadow timer select scroll-bar mldrag mouse jit-lock
font-lock syntax facemenu font-core frame cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
greek romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev loaddefs button
minibuffer faces cus-face text-properties overlay md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind font-render-setting gtk x-toolkit x
multi-tty emacs)