Merge a subset of r1363440 from trunk:

mpm_worker: Fix cases where the spawn rate wasn't reduced after
child process resource shortages.
(Note: mpm_event is omitted from this backport proposal because of
 missing prerequisite fixes.  Even mpm_worker has a tiny issue,
described in comments in the code.)

Submitted by: trawick
Reviewed by: rjung, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1373342 13f79535-47bb-0310-9956-ffa450edef68