If the pdeath signal is set after the synchro we have a window where
the parent exits with the pdeath signal not set.

In order to avoid that, we have to move the prctl before the synchro with
the parent so if the parent exits before we can set the pdeath signal, the
synchro will fail in any case and the container startup will be aborted.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>