The new global keywords "http-err-codes" and "http-fail-codes" allow to
redefine which HTTP status codes indicate a client-induced error or a
server error, as tracked by stick-table counters. This is only done
globally, though everything was done so that it could easily be extended
to a per-proxy mechanism if there was a real need for this (but it would
eat quite more RAM then).
A simple reg-test was added (http-err-fail.vtc).
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