follow up r791309 with additional changes to accommodate the new MPM interfaces

(some old API variables are now private, support retrieval of the MPM generation, 
implement the mpm_note_child_killed hook)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@831951 13f79535-47bb-0310-9956-ffa450edef68