Add a break_hardlinks parameter for tbz2 recompose methods, and enable it

Add a break_hardlinks parameter for tbz2 recompose methods, and enable it
by default. Since bindbapi.aux_update() calls this method, this changes
the default behavior for all modifications to binary package metadata.
This makes it safe to use hardlinks to create cheap snapshots of the
repository, which is useful for solving race conditions on binhosts as
described here:

  http://code.google.com/p/chromium-os/issues/detail?id=3225.