AdHoc will also need the same functionality to verify and parse the
key sequence from GET_KEY. This block of code was moved from AP's
GET_KEY callback into nl80211_parse_get_key_seq.
Netdev/AP share several NL80211 commands and each has their own
builder API's. These were moved into a common file nl80211_util.[ch].
A helper was added to AP for building NEW_STATION to make the associate
callback look cleaner (rather than manually building NEW_STATION).