Add dired-do-compress-to command bound to "c"

Add dired-do-compress-to command bound to "c"

* lisp/dired-aux.el (dired-shell-command): Use the caller's
  `default-directory', return the result of `process-file'.
(dired-compress-file-suffixes): Add comment on why "tar -zxf" isn't
used by default.
(dired-compress-files-alist): New defvar.
(dired-do-compress-to): New command.

* lisp/dired.el (dired-mode-map): Bind `dired-do-compress-to' to "c".
(dired-do-compress-to): Add an autoload entry.

* etc/NEWS: Add two entries.