Run non-daemons services in a different process group to avoid SIGHUP at boot

Run non-daemons services in a different process group to avoid SIGHUP at boot

12 factors apps and similar don't daemonize and are thus vulnerable to
receiving a SIGHUP signal at the end of /etc/rc.  Shield them by running
them in a different process group.  Do this only for services that need
rc_bg=Yes, as suggested by ajacoutot@

There have been several reports about this issue in the past years, the
last one being from edd@ who successfully tested this fix.  Input from
several folks, ok sthen@ ajacoutot@