[PATCH] HTTP Compliance: reply with 504 if required validation failed

HTTP Compliance: reply with 504 (Gateway Timeout) if required validation
failed.

RFC 2616 says that we MUST reply with 504 (Gateway Timeout) if
validation fails and cached reply has proxy-revalidate, must-revalidate
or s-maxage Cache-Control directive.

FwdState::makeConnectingError() method is added to set error status
depending on whether the request was a validation request.

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

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This patch complements an earlier fix that ensured that we forward the
Squid-generated error to the client instead of sending the stale cached
entry when the cached reply has proxy-revalidate, must-revalidate or
s-maxage Cache-Control directive.