Resurrected Duanes old sleep_after_fork patch. This needed or we will

Resurrected Duanes old sleep_after_fork patch. This needed or we will
race with ourselves for swap space when restarting helpers. The hello
test is not really sufficient, we must also wait for the child to actually
exec() before forking again. In the event that we are starting many
helpers who uses CPU it is not too unlikely to see a newly started child
to be context switched before exec().

Anyway, default configuration does nothing. To activate this one must
configure sleep_after_fork in squid.conf so it is a quite harmless patch,
and if it helps some people, fine.