As mentioned inline in comments, correctly handle more sophisticated

As mentioned inline in comments, correctly handle more sophisticated
transformations which currently fail for balancer://foo targets, but
work just fine with other ProxyReverse targets.

  The balancer comparison is a bit trickier.  Given the context

    BalancerMember balancer://alias http://example.com/foo
    ProxyPassReverse /bash balancer://alias/bar

  translate url http://example.com/foo/bar/that to /bash/that

E.g. there may be several different url-suffixes (1st order) of any
particular BalancerMember set e.g. /app1, /app1 and /appbeta while
there may be additional suffixes associated with the actual
ProxyPassReverse directive.  Neither were properly reversed, now
both should be properly handled.

One *critical* assumption;

    BalancerMember balancer://alias/foo http://example.com/bar

should be documented as a meaningless construct, since one cannot
have two members, balancer://alias/foo and balancer://alias/bar,
and the balancer member structures discard this path.

Note one more existing error case as an XXX comment due to invalid
uri comparisons.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@771587 13f79535-47bb-0310-9956-ffa450edef68