BUG/MINOR: mworker: don't use oldpids[] anymore for reload

BUG/MINOR: mworker: don't use oldpids[] anymore for reload

Since commit 3f12887 ("MINOR: mworker: don't use children variable
anymore"), the oldpids array is not used anymore to generate the new -sf
parameters. So we don't need to set nb_oldpids to 0 during the first
start of the master process.

This patch fixes a bug when 2 masters process tries to synchronize their
peers, there is a small chances that it won't work because nb_oldpids
equals 0.

Should be backported as far as 2.0.