[PATCH] immortal helpers

This patch adds an advanced option to the Squid helper controls which
overrides Squid dying when helpers crash.

It has been found necessary in certain corner cases with PHP helpers
(which have system imposed limited lifetimes) where a proxy has
previously been under some load and helpers started then are timed out
later under low load as a bunch. Squid may die and restart.

If the proxy has been started under existing high load conditions (such
as a backup coming online) this case may also occur shortly after
startup. Leading to a chain reaction of restarts until load drops below
number of helpers needed to trigger a death.

These cases depend on external forces or helper design closing the
helpers outside Squid control.

There is one known issue with this option:

   If the helpers are really dying due to some fatal issue during their
startup the use of this option would result in Squid hanging while
infinitely re-starting helpers and doing no request processing.

Amos