addr_adv plugin: skip link-local addresses by default

[matttbe]

Is your feature request related to a problem? Please describe.
Because the in-kernel PM supports a maximum of 8 local endpoints, it is probably best to avoid using the LL ones by default as it should fit best what would normally be used.

Describe the solution you'd like
Set this by default in the config file:

notify-flags=existing,skip_link_local,skip_loopback

And maybe set them by default if notify-flags is not set? But we still need to accept notify-flags=<empty>

Describe alternatives you've considered
The current config can select too many addresses and create confusions when there are error messages and missing endpoints, see #281

Additional context
see #281

It is more a change of behaviour than a new feature.