add support for expected throughput

cfg80211 allows drivers to announce the to-be-expected layer-2 datarate
using the NL80211_STA_INFO_EXPECTED_THROUGHPUT field.
This information is useful as a metric for user-space routing daemons,
so grab it via nl80211 and make it available in both C and Lua APIs,
and show expected throughput on CLI interface assoclist.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>