[MINOR] startup: don't wait for nothing when no old pid remains

[MINOR] startup: don't wait for nothing when no old pid remains

In case of binding failure during startup, we wait for some time sending
signals to old pids so that they release the ports we need. But if there
aren't any old pids anymore, it's useless to wait, we prefer to fail fast.
Along with this change, we now have the number of old pids really found
in the nb_oldpids variable.