Update AP selection heuristics for auto-join.

Update AP selection heuristics for auto-join.

We now prefer stronger crypto over weaker crypto over none, prefer 5 GHz
band with reasonable RSSI, and use RSSI as a tie-breaker with a slight
advantage for 5GHz. Candidate APs are now ranked by a score which is
calculated based on these attributes.

There is likely room for improvements to make these heuristics
work well across many different environments, but it's a start.

ok phessler@