http: Enforce that fully qualified uri-paths not to be forward-proxied

http: Enforce that fully qualified uri-paths not to be forward-proxied
      have an http(s) scheme, and that the ones to be forward proxied have a
      hostname, per HTTP specifications.

The early checks avoid failing the request later on and thus save cycles
for those invalid cases.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1895921 13f79535-47bb-0310-9956-ffa450edef68