pkg/tarheader: support hard links when building tar headers
pkg/tarheader: support hard links when building tar headers

This introduces an inode "cache" that can be used when populating a tar
header (pkg/tarheader.Populate), so that any inodes already seen can be
linked to instead of inserting an entire copy of the file into the tar.

craimbert/spec,apcera/appc-spec,jonboulle/spec,apcera/appc-spec,3ofcoins/appc-spec,zenlinTechnofreak/spec,ChengTiesheng/spec,krnowak/appc-spec,yifan-gu/spec,alban/appc-spec,stufflebear/spec,squeed/spec,jonboulle/spec,apcera/appc-spec,ChengTiesheng/spec,marineam/spec,mesosphere/spec,tklauser/spec,yifan-gu/spec,jonboulle/spec,krnowak/appc-spec,dustymabe/spec,iaguis/appc-spec,sgotti/spec,3ofcoins/appc-spec,stufflebear/spec,zenlinTechnofreak/spec,endocode/appc-spec,alban/appc-spec,squeed/spec,iaguis/appc-spec,dustymabe/spec,sgotti/spec,endocode/appc-spec,appc/spec,tklauser/spec,krnowak/appc-spec,squeed/spec,dgonyeo/spec,appc/spec,tklauser/spec,dustymabe/spec,philips/spec,philips/spec,yifan-gu/spec,3ofcoins/appc-spec,alban/appc-spec,craimbert/spec,appc/spec,dgonyeo/spec,craimbert/spec,marineam/spec,ChengTiesheng/spec,mesosphere/spec,dgonyeo/spec,endocode/appc-spec,zenlinTechnofreak/spec,iaguis/appc-spec,stufflebear/spec,marineam/spec