Motivation:
* Previous implementation silently failed when an integer overflow
occurred. Now, we report an ErrFieldTooLong.
* Previous implementation did not encode in two's complement format and was
unable to encode negative numbers.

The relevant GNU specification says:
<<<
GNU format uses two's-complement base-256 notation to store values
that do not fit into standard ustar range.
>>>

Fixes #12436

Change-Id: I09c20602eabf8ae3a7e0db35b79440a64bfaf807
Reviewed-on: https://go-review.googlesource.com/17425Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>