Do not hardlink files unless they are in the same

Do not hardlink files unless they are in the same
directory, since otherwise tar may not be able to
extract a tarball of the resulting hardlinks due to
'Invalid cross-device link' errors (depends on layout of
mount points). Also, don't hardlink zero-byte files since
it doesn't save any space. Thanks to Daniel Robbins for
reporting these issues which were noticed by funtoo users
when attempting to extract stage tarballs. (trunk r12381)

svn path=/main/branches/2.1.6/; revision=12451