Bug 3130: helpers are crashing too rapidly

Bug 3130: helpers are crashing too rapidly

As discussed quite many months ago. This reduces the FATAL when helpers
crash/exit to a critical level ERROR have responded with useful reply to
at least one lookup.

The result is that Squid can now cope with helpers written in languages
which cannot loop infinitely. For example; PHP helpers often exit after
a timeout, broken scripts written to respond and exit immediately,
and helpers which encounter some permissions error and respond only with
"ERR" or "BH" results before aborting.