Fix a hang condition with graceful restart and prefork MPM

Fix a hang condition with graceful restart and prefork MPM
in the situation where MaxClients is very high but
much fewer servers are actually started at the time of the
restart.

The way we notify an entire generation to die at once is
changed so that we don't have to use the pod (and deal with
the ease of filling the kernel pipe buffer).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93999 13f79535-47bb-0310-9956-ffa450edef68