Correctly parse an IPv6 literal host specification in an absolute URL

Correctly parse an IPv6 literal host specification in an absolute URL
in the request line.

- Fix handling of brackets [ ] surrounding the IPv6 address.
- Skip parsing r->hostname again if not necessary.
- Do some checks that the IPv6 address is sane. This is not done by
  apr_parse_addr_port().

log client error at level debug, log broken Host header value

Backports: r1407006, r1426827
Submitted by: sf

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-merge-http-strict@1768035 13f79535-47bb-0310-9956-ffa450edef68