[Bug-tar] Errors in "posix" format.

[Tim Kientzle]

I've been doing some interoperability testing between bsdtar
and gtar's newly-implemented "posix" format and I have a few quibbles:

 * No user/group stored with 'x' entry.
   If the archive is extracted by a reader that
   doesn't understand pax extended attributes,
   then the 'x' entry is supposed to be treated
   as a regular file.  (According to POSIX, since
   about 1987.)  It's therefore important that the
   'x' entry have basic permission and ownership
   attributes set correctly.

 * Poor naming of 'x' entry.   Long filenames get
   truncated at 100 characters.  Following the logic
   above, this could result in readers overwriting
   other files with the contents of 'x' entries.

 * No size stored in main entry if file size exceeds
   8gig.  Readers that don't support pax extended attributes
   rely on the file size to locate the next entry.
   It is quite appropriate to use numeric extensions
   (e.g., base-256) here.  Compliant readers should
   ignore the value (in favor of the value stored in the
   extended attribute entry), so it does not compromise
   standards compliance, but does help ensure that
   the widest possible array of readers can extract
   the file (such as GNU tar 1.13 ;-).

Finally, I find the name "posix" for this format to
be quite misleading; some people will think you're referring
to "ustar" which is also a POSIX format.  The official
name of this format in the Single Unix Standard is "pax extended."
For reference, you might take a close look at two other open-source
archivers that implement this format:  star refers to this format
as "xustar" (extended ustar) and bsdtar refers to this as "pax".

Tim Kientzle