Pull up following revision(s) (requested by ozaki-r in ticket #911):

sys/kern/init_main.c: revision 1.498
sys/rump/net/lib/libnet/net_component.c: revision 1.10
sys/net/if.h: revision 1.264
sys/net/if.c: revision 1.429

Fix net.inet6.ip6.ifq node doesn't exist

The node (and child nodes) is initialized in sysctl_net_pktq_setup, but the call
of sysctl_net_pktq_setup is skipped unexpectedly.
sysctl_net_pktq_setup is skipped if in6_present is false that indicates the
netinet6 component isn't loaded on rump kernels.  However the flag is
accidentally always false because the flag is turned on in in6_dom_init that is
called after if_sysctl_setup on both normal and rump kernels.

Fix the issue by moving if_sysctl_setup after in6_dom_init (domaininit on normal
kernels).  This fix is ad-hoc but good enough for netbsd-8.  We should refine
the initialization order of network components in the future.

Pointed out by hikaru@

(martin)