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.

svn path=/main/trunk/; revision=12381