Fix return values from wait_for_many_objects.

The return value is index to the signaled thread in
the creted_threads array.
We can not use WAIT_TIMEOUT as return value
because its value is defined as 258, thus limiting
the MaxThreads to that value that leads to the
assertion errors.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@428029 13f79535-47bb-0310-9956-ffa450edef68