Some extra tweaks to the DAV code file/path handling (to continue the

platforms fixes done by wrowe).

* dav_fs_get_parent_resource(): return NULL if we're at the root of
    the URL (Location) space.

* dav_fs_dir_file_name(): make it return an error, in case we're
    passed a bad path. adjusted callers [who just ignore it for now]


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93009 13f79535-47bb-0310-9956-ffa450edef68