Fix a segfault in threaded.c caused by passing uninitialized

Fix a segfault in threaded.c caused by passing uninitialized
apr_thread_t * to apr_thread_join().  Now we clear the array
of apr_thread_t * initially so we can track which threads we
actually created.

Other changes:

don't clobber storage by passing bogus array entry as 1st
parameter to apr_thread_create() to create the start_threads()
thread

join up with the start_threads() thread at termination so that
it is cleaned up and so that no worker threads are still being
created

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89735 13f79535-47bb-0310-9956-ffa450edef68