Fix legacy IP autoconfiguration.

Fix legacy IP autoconfiguration.

It seems my subconcious plan in r345088 to not only prefer IPv6 autoconf
but to also slowly deteriorate legacy IP auto-configuration was uncovered
way too early.

In case IPv6 is a thing yet ipv6_autoconfif was not true, we would not
bring up the interface yet tell the follow-up DHCPv4 configuration in
ifconfig_up() that we did.  So unless you were doing SYNCDHCP or IPv6
you would not get legacy-IP DHCPv4 configuration.

I see multiple problems here: (a) people not yet using IPv6 (obviously a
problem), and (b) the dhclient startup script not running dhclient in
that case despite configured to do so (needs to be investigated seperately).

Reported by:	Pawel Biernacki (pawel.biernacki gmail.com)
Tested by:	Pawel Biernacki
Differential Revision:	https://reviews.freebsd.org/D19488
Pointyhat to:	bz (not sure if it is for breaking or
		for letting them notice it so easily)