apr_put_os_file() expected the caller to have an existing file or init to

apr_put_os_file() expected the caller to have an existing file or init to
NULL. using an existing file doesn't normally work: where would you get a
blank file to shove an FD into? expecting the user to assign to NULL is
error-prone (mod_isapi didn't).

*) always create and return a new file from apr_put_os_file()
*) reimplement apr_open_stderr() in terms of apr_put_os_file()
   [ except for win32... some issues there ]
*) remove some (obsolete) inits to NULL

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87851 13f79535-47bb-0310-9956-ffa450edef68