We often want to be pedantic about not accepting %NULL for getters (or ref,
unref, etc). Often that is also inconvenient, so we would need to write:

  if (l3cd)
      strv = nm_l3_config_data_get_nameservers(l3cd, addr_family, &len);
  else
      len = 0;

(and, make sure that strv does not trigger a maybe-uninitialized warning).

Being pedanic here is more cumbersome than helpful. Accept NULL to return
the sensible default.

Also add nm_l3_config_data_get_dns_priority_or_default() helper which maps
NULL or a missing value to zero. This is also only for convenience for certain
callers.

The plain nm_strv_equal(), nm_strv_equal_n(), nm_strv_cmp_n() functions
treat NULL strv arrays (without specified length) different than strv
arrays of length zero. They do so, because that is useful sometimes.

Sometimes it is not useful and we want to treat empty arrays the same
as a NULL array. Add helpers for that.

Completely rework IP configuration in the daemon. Use NML3Cfg as layer 3
manager for the IP configuration of an interface. Use NML3ConfigData as
pieces of configuration that the various components collect and
configure. NMDevice is managing most of the IP configuration at a higher
level, that is, it starts DHCP and other IP methods. Rework the state
handling there.

This is a huge rework of how NetworkManager daemon handles IP
configuration. Some fallout is to be expected.

It appears the patch deletes many lines of code. That is not accurate, because
you also have to count the files `src/core/nm-l3*`, which were unused previously.

Co-authored-by: Beniamino Galvani <bgalvani@redhat.com>

invoking nmcli command without arguments will now show "default"
instead of null adress in route4 or route6 section.