BUG/MINOR: mworker/init: don't reset nb_oldpids in non-mworker cases

BUG/MINOR: mworker/init: don't reset nb_oldpids in non-mworker cases

This bug affects the peers synchronisation code which rely on the
nb_oldpids variable to synchronize the peer from the old PID.

In the case the process is not started in master-worker mode and tries
to synchronize using the peers, there is a small chance that won't work
because nb_oldpids equals 0.

Fix the bug by setting the variable to 0 only in the case of the
master-worker when not reloaded.

It could also be a problem when trying to synchronize the peers between
2 masters process which should be fixed in another patch.

Bug exists since commit 8a361b5 ("BUG/MEDIUM: mworker: don't reuse PIDs
passed to the master").

Sould be backported as far as 1.8.