Merge r1363183 from trunk:

mpm_prefork: Reduce spawn rate after a child process exits due to
unexpected poll or accept failure.

Submitted by: trawick
Reviewed by:  jorton, rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1366515 13f79535-47bb-0310-9956-ffa450edef68