If several directories are given in a UserDir directive, only files in the first...

If several directories are given in a UserDir directive, only files in the first existing one are checked. If the file is not found there, the other possible directories are not checked. The doc clearly states that they will be checked one by one, until a match is found or an external redirect is performed.

 PR 59636.

While at it, add some debug messages to better understand what is performed.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1832306 13f79535-47bb-0310-9956-ffa450edef68