Prior to the dire warning about values of network_interfaces other than

AUTO the biggest mistake users made was leaving lo0 off the list. Since
lo0 is effectively mandatory, check for it and add it to the list if
it's not there.

etc/network.subr

@@ -727,6 +727,13 @@ list_net_interfaces()
 		;;
 	*)
 		_tmplist="${network_interfaces} ${cloned_interfaces}"
+
+		# lo0 is effectively mandatory, so help prevent foot-shooting
+		#
+		case "$_tmplist" in
+		lo0*|*lo0|*' lo0 '*)	;;	# This is fine, do nothing
+		*)	_tmplist="lo0 ${_tmplist}" ;;
+		esac
 		;;
 	esac