Support hardlinks in files added to RPMs

[jvanzyl]

Is it possible for RPMs to support hardlinks in the underlying CPIO implementation efficiently? I’m happy to submit a PR if this is possible but I wasn’t sure from reading the CPIO spec if hardlinks can be efficiently stored or if the content is just duplicated. Supporting the creation of hardlinks during installation of the RPM is not what I'm looking for. With a distribution I'm looking at the tar.gz file can be reduced by 2/3rds using hardlinks and I'm looking for the same with the RPM.