fix a problem in the threaded and worker mpm's where a keepalive connection

on one thread can prevent the process from exiting.

let's rename ap_graceful_restart_signalled to something more appropriate
after the next beta.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89881 13f79535-47bb-0310-9956-ffa450edef68