prefork MPM: add -DFOREGROUND option to use when you want

             the parent process to run in the foreground

             (NO_DETACH is a special mode for running under
             programs like daemontools)

             the existing flag -DDEBUG turns on FOREGROUND
             now instead of NO_DETACH


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94419 13f79535-47bb-0310-9956-ffa450edef68