- merge the pointless helper function vlan_priorities_to_string()
  into the only caller _get_fcn_vlan_xgress_priority_map().

- minor cleanups, like setting out-is-default if num==0, not
  based on whether we have a non-empty string. There is not difference
  in practice, because nm_setting_vlan_get_priority() never fails.
  Hence they are identical. But nm_setting_vlan_get_priority() has
  an API that allows it to fail, so we should declare the default
  depending on the number of vlan priorities.

- don't allocate the temporary GString instance if we won't need it.

- only append the delimiter if needed, and not truncate it afterwards.
  It might have even worse performance this way, but it feels more
  correct to me.

- also cache the result of nm_setting_vlan_get_num_priorities().
  NMSettingVlan's implementation is horrible and uses a GSList to
  track the list of priorities. This makes it relatively expensive
  to call get-num-priorities repeatedly (and pointless).

(cherry picked from commit bbfd3668)