Prefer 5 GHz networks over 2.4 GHz networks

Prefer 5 GHz networks over 2.4 GHz networks

In scan.c, merge a channel's noise value into the scan results. When
comparing scan results, compute the signal-to-noise ratio and use it
when available. Prefer a 5 GHz network if its SNR is really big (> 30)
or if its SNR is relatively close to the other network's.