Merge 1387633,1392850 from trunk:

* server/mpm/prefork/prefork.c (child_main): Don't log errors for an
  apr_pollset_add() failure if a graceful-stop has been signalled.

follow up to r1387633: use the right exit code depending on the error scenario

Submitted by: jorton, trawick
Reviewed by: trawick, minfrin, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1393382 13f79535-47bb-0310-9956-ffa450edef68