ACHANNELS 1 hides a channel of the next device (still)

[jmkristian]

This follows on to issue #510. If I configure two mono devices, like this:

ADEVICE plughw:CARD=Digirig,DEV=0
ACHANNELS 1
ADEVICE1 plughw:CARD=DRA,DEV=0
ACHANNELS 1

... the second device isn't visible to AGW clients. For example, QtTermTCP shows:

It appears the second AGW port represents the second device, but actually it corresponds to channel 1. An AGW client can't use this port, as shown in the attached log.
mono2.log

In this case, either Direwolf should map the second AGW port to channel 2, or the 'G' response should identify three AGW ports (representing channels 0, 1 and 2). I would prefer the former; that is, AGW ports don't map to invalid channels.

I built direwolf on Ubuntu 22.04.3 from the dev branch commit 4d2d814.