bug#19481: package.el: support .tar archives featuring a pax_global_header entry

[Eli Zaretskii]

> From: Ivan Shmakov <ivan@siamics.net>
> Date: Wed, 28 Jan 2015 20:54:33 +0000
> 
>       Please consider the revised patch MIMEd.
> 
>       * lisp/tar-mode.el (tar-header-extractable-p): New function.
>       (tar-untar-buffer): Use it; or use the value of the new optional
>       argument instead.
> 
>       * lisp/emacs-lisp/package.el (package-untar-buffer): Use
>       tar-header-extractable-p.
>       (package-tar-file-info): Consider the second file name in the
>       archive if the first one has no directory component.

Thanks.

Can we have a test suite for this file, and in particular for these
changes?  Failing that, please describe how you tested these changes.

Otherwise, I have no comments, and no objections to commit this, once
we are sure it doesn't break anything.