Replaces PID in PaxHeaders with 0.  Sorts PAX header keys before writing
them to the archive.

Fixes #12358

Change-Id: If239f89c85f1c9d9895a253fb06a47ad44960124
Reviewed-on: https://go-review.googlesource.com/13975Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>