IPv6: rc boot time improvements

[bz]

Summary

This is a first cut to improve boot time for IPv6.
Hisotrically we did sleep quite a lot more than we needed.

If IPv6 is enabled we call rtsol on that interface so waiting in
rtsold is counterproductive; if one was to just use rtsold -F then
they are on their own for having the addresses usable during the
remaining startup time.

Rather than sleeping dad_count + 1 seconds we are now checking for
any tentative addresses on the interface and only then slee
dad_count up-to 2 times. This should not save us anything with the
defaults but there are too many assumptions invovled anyway
(RETRANS_TIMER in the kernel being 1000ms being one).
The reason why we need to sleep more than dad_count (1s default)
seems to be that first DAD for the link-local address clears and
only then solicited (or other global) addresses are handled.

[bz]

Just a first cut for your testing to see what difference it makes?
I would assume you'd sleep 2 rather than 4 seconds.

I'll add reviewers once we have initial results.

libexec/rc/network.subr

51

This is unrelated for this and should be a follow-up to an old commit I once did ... (lookup hash and factor out here). Should not matter anyway.

1749

This should be done separately as well; we are only interested in inet6 addresses so avoid printing a load of other stuff.

[bz]

More unrelated noise in the change.

libexec/rc/rc.d/netif

228

Also unrelated: moving the interfaces between vnets does not need DAD as we should never move addresses (link-local maybe on the vnetdown case) but really interface address configuration in that case is what matters and not the interface appearing.

[cperciva]

On the IPv4 side, when /etc/rc.d/dhclient runs, it gets an IPv4 address before the dhclient process is backgrounded and the boot process continues; so anything which comes later in the boot process is guaranteed to see the necessary addresses.

On the IPv6 side, I don't think we have any sort of deliberate wait-for-addresses-to-arrive... except that we might be getting that accidentally thanks to the extra sleeping here?