Merge r1711479, r1733064, r1733068 from trunk:

Fix crash in ap_mpm_pod_check call caused by NULL dereference of its parameter
when starting httpd as single process (httpd -X).


Revert changes on mpm_event and mpm_worker from r1711479.
The POD is not used in one process mode for those MPMs.

Follow up to r1711479 and r1733064: CHANGES entry.
Submitted by: jkaluza, ylavic, ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1733475 13f79535-47bb-0310-9956-ffa450edef68