Medium: cib: Allow backup files to proceed after wrap-around (bnc#630322)

The cib did not unlink the target for the backup files, so the link()
would fail once the maximum number of files was reached. (link does not
overwrite.) Found and fixed by Michal Srb.

Mercurial revision: dfc6b256137317985b4e3ca31373edb93bb1d076