mpm_prefork: mask signals during ap_run_child_init(). This prevents threads potentially created from the child_init hooks (e.g. mod_watchdog workers) to catch signals needed by the MPM, like here: https://travis-ci.com/github/apache/httpd/jobs/510821148#L5356.
httpd/httpd/trunk/changes-entries/prefork_child_init_sigmask.txt