Add support for PAX subsecond resolution times. Since the parser
supports negative timestamps, the formatter also handles negative
timestamps.

The relevant PAX specification is:
<<<
Portable file timestamps cannot be negative. If pax encounters a
file with a negative timestamp in copy or write mode, it can reject
the file, substitute a non-negative timestamp, or generate a
non-portable timestamp with a leading '-'.
>
> <<<
> All of these time records shall be formatted as a decimal
> representation of the time in seconds since the Epoch.
> If a <period> ( '.' ) decimal point character is present,
> the digits to the right of the point shall represent the units of
> a subsecond timing granularity, where the first digit is tenths of
> a second and each subsequent digit is a tenth of the previous digit.

Fixes #11171

Change-Id: Ied108f3d2654390bc1b0ddd66a4081c2b83e490b
Reviewed-on: https://go-review.googlesource.com/55552
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>