The proper fix for the Writer is too involved to be done in time
for Go 1.8. Instead, we do a localized fix that simply disables the
prefix encoding logic. While this will prevent some legitimate uses
of prefix, it will ensure that we don't keep outputting invalid
GNU format files that have the prefix field populated.

For headers with long filenames that could have used the prefix field,
they will be promoted to use the PAX format, which ensures that we
will still be able to encode all headers that we were able to do before.

Updates #12594
Fixes #17630
Fixes #9683

Change-Id: Ia97b524ac69865390e2ae8bb0dfb664d40a05add
Reviewed-on: https://go-review.googlesource.com/32234Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>