Fix positional arg usage of AudioSource.

The sample config files used a positional arg for AudioSource's
device name, but the first positional arg is actually the block name.
Change the order to match this usage.
Use band record modes to pick receiver modes if needed.

That is, if a mode has not been specified, and any frequency DB entry
contains the target frequency, use the mode from that entry.

Additionally, don't fail to create a receiver if the mode specified is
unknown.
Don't create a new receiver on click when the mode is different.

This bit of DWIM has always been more surprising than useful to me, and
it now has an especially bad interaction with a band marked with an
unsupported mode (creating receivers on every click).