Improve parsing of the Host by following RFC 7230 Section 5.4 more strictly:

Improve parsing of the Host by following RFC 7230 Section 5.4 more strictly:

- Respond with a 400 (Bad Request) if there is more than one Host:
header to prevent ambiguities.

- Make sure that the host in the optional absolute form of
request-target (eg. GET http://www.target.com/ HTTP/1.1) matches the
Host: value.  Proxies are supposed to ignore the Host: value if the
request-target exists, but relayd used to ignore the absolute
request-target form instead.  In HTTP terminology, relayd is a gateway
and not a proxy, but it has to make sure that the host is validated
consistently.

OK benno@ bluhm@