. unfortunately we can't use enum iw_mgmt_info_element_ids for the
  id field in iw_mgmt_info_element, as it has to be u8 and sizeof(enum)
  is bigger than that, but we use the enum in the relevant functions to
  help catch invalid elements being used.
. also we can't have iw_mgmt_info_element with a fixed size data field,
  as it is variable as per the 802.11 specs, so I do a poor man's OOP
  by subclassing iw_mgmt_info_element into the standard element types. Done
  up to now with iw_mgmt_essid_pset and iw_mgmt_ds_pset, others will follow.