mod_proxy: log diagnostics during ProxyPass[Match]

To help out users when debugging ProxyPass and ProxyPassMatch, log all
match attempts (at trace2), as well as matches that are either
successful or explicitly disabled (at trace1).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1762517 13f79535-47bb-0310-9956-ffa450edef68