BibSched: fix signal propagation to spawned tasks
BibSched: fix signal propagation to spawned tasks

* Commit 23e2e2b5a5a2ffc569048ee578bc8dfaeed2615e introduced
  an improved bibtask spawning technique that should have
  prevented bibtasks from receiving signals sent to the spawner
  bibsched daemon. This was actually not the case.
  This commit should fix the bug.

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio