prefer 5ghz over 2ghz

[bonds]

11a performs better than 11g, at least in my tests. Give 11a a 20% advantage, as long as its above the minimum, when choosing the strongest network.

[bonds]

Can detect by chan >= 36. Let's make it into a setting, i.e. prefer_5ghz, defaults to false, causes winot to use 5ghz if any are >= 30% strength.