archive/tar: automatically promote TypeRegA

Change Reader to promote TypeRegA to TypeReg in headers, unless their
name have a trailing slash which is already promoted to TypeDir. This
will allow client code to handle just TypeReg instead both TypeReg and
TypeRegA.

Change Writer to promote TypeRegA to TypeReg or TypeDir in the headers
depending on whether the name has a trailing slash. This normalization
is motivated by the specification (in pax(1)):

   0 represents a regular file. For backwards-compatibility, a
   typeflag value of binary zero ( '\0' ) should be recognized as
   meaning a regular file when extracting files from the
   archive. Archives written with this version of the archive file
   format create regular files with a typeflag value of the
   ISO/IEC 646:1991 standard IRV '0'.

Fixes #22768.

Change-Id: I149ec55824580d446cdde5a0d7a0457ad7b03466
Reviewed-on: https://go-review.googlesource.com/85656


Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>