bin/52618: rtadvd fails when empty vlan interface exists

[clare%csel.org@localhost]

>Number:         52618
>Category:       bin
>Synopsis:       rtadvd fails when empty vlan interface exists
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Oct 14 17:25:00 +0000 2017
>Originator:     Shinichi Doyashiki
>Release:        NetBSD 8.99.1
>Organization:
	at home
>Environment:
System: NetBSD inner.csel.org 8.99.1 NetBSD 8.99.1 (APU1C) #6: Sun Jul 9 20:25:50 JST 2017 clare%kanako.csel.org@localhost:/export/stage/hack/sys/arch/amd64/compile/APU1C amd64
Architecture: x86_64
Machine: amd64
>Description:
	rtadvd failes when empty vlan interface exists
>How-To-Repeat:
	1. configure empty vlan interface
	# touch /etc/ifconfig.vlan2
	2. configure rtadvd with empty interface, and start
	# vi /etc/rc.conf
	# grep rtadvd /etc/rc.conf
	rtadvd=YES; rtadvd_flags="re1 vlan2 vlan3 vlan10 vlan11"
	# reboot
	3. watch /var/log/messages
	Oct 15 02:07:55 inner rtadvd[972]: <sock_open> IPV6_JOIN_GROUP(link) on vlan2: Invalid argument
>Fix:
	workaround: keep interface(s) always configured condition.