Suppress 'underground' if 'underground_rf' in use.

Previously if you have 'underground' in your [logged] services and
'underground_rf' (RapidFire) in your [live] services then both types of
upload would be used. Weather Underground don't say not to do this, but
it is an overhead sending near-duplicate data.

Now, by careful fiddling with 'last update' timestamps, 'underground' is
only used for catchup if your station goes off line for any reason. Once
the catchup data has been sent only 'underground_rf' is used.