HTTP Compliance: Reply with an error if required validation fails.

HTTP Compliance: Reply with an error if required validation fails.

RFC 2616 says that proxy MUST not use stale entries that have s-maxage,
proxy-revalidate, or must-revalidate cache-directive.

Add new fail_on_validation_err request flag to store result from
refreshCheck().  It is needed to avoid refreshLimits() recalculation in
clientReplyContext::handleIMSReply().

Split LOG_TCP_REFRESH_FAIL into LOG_TCP_REFRESH_FAIL_OLD (stale reply sent)
and LOG_TCP_REFRESH_FAIL_ERR (error forwarded). However, both are still logged
as TCP_REFRESH_FAIL for backward-compatibility with external scripts and such.
We may decide to start logging more detailed codes later.

Co-Advisor test cases:
    test_case/rfc2616/noSrv-hit-must-reval-s-maxage-resp
    test_case/rfc2616/noSrv-hit-must-reval-proxy-revalidate-resp
    test_case/rfc2616/noSrv-hit-must-reval-must-revalidate-resp