Add '_nm_setting_bond_get_option_or_default()' and move all the custom
policies applied by NM for bond options in there.
One such example of a custom policy is to set 'miimon' to 0 (instead of its
default value of 100) if 'arp_interval' is explicitly enabled
and 'miimon' is not.
This means removing every piece of logic from
nm_setting_bond_add_option() which used to clear out 'arp_interval' and
'arp_ip_target' if 'miimon' was set or clear out 'miimon' along with
'downdelay', 'updelay' and 'miimon' if 'arp_interval' was set.
This behaviour is a bug since the kernel allow setting any combination
of this options for bonds and NetworkManager should not limit the user
to do so.
Also use 'set_bond_attr_or_default()' instead of 'set_bond_attr()' as
the former calls '_nm_setting_bond_get_option_or_default()' to implement
the right logic to retrieve bond options according to current bond
configuration.
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