Fix the prefix_stat function. (which does a stat call on the first

Fix the prefix_stat function. (which does a stat call on the first
path segment). This function was still tailored for unix systems only.
It should work on other systems as well now.

Note that in 2.x we use the apr_filepath_root function that doesn't
exist in 1.3. It is replaced by some simple workaround stuff which
behaves similar.

Obtained from: 2.1 (r1.143)
Reviewed by: Brad Nicholes, Will Rowe

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@99178 13f79535-47bb-0310-9956-ffa450edef68