commit db488c79 assumed that dnsmasq would complete IPv6 DAD before
daemonizing, but in reality it doesn't wait, which creates problems
when libvirt's bridge driver sets the matching "dummy tap device" to
IFF_DOWN prior to DAD completing.

This patch waits for DAD completion by periodically polling the kernel
using netlink to check whether there are any IPv6 addresses assigned
to bridge which have a 'tentative' state (if there are any in this
state, then DAD hasn't yet finished). After DAD is finished, execution
continues. To avoid an endless hang in case something was wrong with
the kernel's DAD, we wait a maximum of 5 seconds.