Increase security in core.c by testing (as we merge the path) that the
  URI does not go above the DocumentRoot (as defined by the OS, not by
  the URI specification), and give us the true name.

  When we are done, note the name is canonical for directory_walk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90593 13f79535-47bb-0310-9956-ffa450edef68