Support reading pax archives and applying extended attributes
like long names, subsecond mtime resolutions, etc. Default to
writing pax archives for long file and link names.
Support reading gnu archives using the ././@LongLink extended
header for file name and link target.

Fixes #3300

R=dsymonds, dave, rogpeppe, remyoudompheng, chressie, rsc
CC=golang-dev
https://golang.org/cl/6700047