archive/tar: read/write extended pax/gnu tar archives

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 issue 3300

R=dsymonds, dave, rogpeppe, remyoudompheng, chressie, rsc
CC=golang-dev
https://codereview.appspot.com/6700047

Committer: David Symonds <dsymonds@golang.org>
doc: update contribute.html

Fixes issue 4582.

* mentioned hg upload.
* added section on hg file.
* added small mention about being inside $GOROOT.
* added hg revert @NNNN
* reorganise the hg submit section for the common case of a non committer.
* made the Copyright section h2
* added note about leaving copyright years unchanged.

R=golang-dev, metanata4, shivakumar.gn, minux.ma, adg, shanemhansen
CC=golang-dev, metanata4
https://codereview.appspot.com/7278047