Add child_status hook for tracking creation/termination of MPM child

processes.  Add end_generation hook for notification when the last
MPM child of a generation exits.

end_generation is implemented completely by core using the
child_status hook run by the MPM.

simple and mpmt_os2 MPMs don't currently run the child_status
hook, so neither hook is invoked with those MPMs.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1096609 13f79535-47bb-0310-9956-ffa450edef68