- the previous implementation of nm_setting_wired_get_s390_option()
  returned the elements in an arbitrary order (because it just iterated
  idx times over the unsorted hash table).

- the API for "s390-options" suggests both accessing by index and by
  name. Storing the options in a hash-table is not optimal for lookup
  by index. It also requires us to sort the elements over and over
  again.
  Use instead a sorted array. Note that add/remove of course requires to
  move the elements (and has thus O(n)).

- "s390-options" are very seldomly set. We shouldn't pay the price in every
  NMSettingWired to allocate a GHashTable and deal with it.

- don't assert in nm_setting_wired_add_s390_option() and
  nm_setting_wired_remove_s390_option() that the key is valid.
  ifcfg-rh reader understandably does not want to implement additional
  logic to pre-validate the key, so any invalid keys would trigger an
  assertion failure. We have verify() for this purpose.