ap_start_shutdown is not used by anybody outside of the MPMs, so we don't

need to put it in the ap_mpm.h header file.  This also makes all of the
instances of ap_start_shutdown static.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87428 13f79535-47bb-0310-9956-ffa450edef68