BUG/MINOR: mworker: don't save program PIDs in oldpids

BUG/MINOR: mworker: don't save program PIDs in oldpids

After reload, previously launched programs are stopped explicitly in
mworker_ext_launch_all(). So, there is no longer need to save their PIDs in
oldpids array before the master reexec().

This also prepares the fix of "-D -W -sf/-st" modes, as we will need to
loop over this array in the master process context, in order to stop the
previous master, when the new one is ready.

This patch should be backported only in 3.1.