Q module: Store default NVs and better default host cloak behavior

See #525 for reference.

I've tested this, and this only affects users who load the Q module for the first time while connected to IRC.

If the Q module is already loaded (existing users), ZNC is upgraded and they connect to IRC: they get +x set, UseCloakedHost is stored as true in their config and they will never notice anything changed.

If the Q module wasn't already loaded, and the user is connected to IRC, they will be notified that they can cloak now if they want, or change the default setting (to keep their host).

What do you think?