device: prevent IP state from reaching ready while DNS update pending

The goal would be to ensure that a device cannot move to activated,
while a DNS update is still pending.

This does not really work for most cases. That is, because NMDevice does
not directly push DNS updates to NMDnsManager, instead, NMPolicy is
watching all device changes, and doing it. But when NMPolicy decides to
to that, may not be the right moment.

We really should let NMDevice (or better, NML3Cfg) directly talk to
NMDnsManager. Why not? They have all the information when new DNS
configuration is available. The only thing that NMPolicy does on top of
that, is determining which device has the best default route. NMPolicy
could continue to do that (or maybe NMDnsManager could), but the update
needs to be directly triggered by NMDevice/NML3Cfg.