mpm_unix(es): cleanup properly on exit in one_process mode.

We can destroy ap_pglobal because clean_child_exit() runs in DSO which would be
unloaded under us, so use atexit() to differ the final apr_terminate().



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1814112 13f79535-47bb-0310-9956-ffa450edef68