Compliance: reply with 400 (Bad Request) if request header is too big

Compliance: reply with 400 (Bad Request) if request header is too big.

Reply with a standard 400 (Bad Request) instead of 601 (Unknown) status
in case of an ERR_TOO_BIG error. HTTP does not have a dedicated code for
the too-big header error. There is 414 (Request-URI Too Long), but Squid
does not distinguish too-large headers from too-large URIs.

Co-Advisor test case: test_case/rfc2616/longUri-65536