Set --ch-index to a newly added channel when --ch-add is set, to allow further modification

[ianmcorvidae]

This isn't quite the fix as described in the issue, but I believe fixes #507 a bit more nicely even than that.

Currently, if you were to pass all of --ch-add, --ch-index, and --ch-set, the CLI would first add a channel at the first available index, but then the --ch-set would apply to whatever channel was at --ch-index, which needn't necessarily match the channel that was just added. That is, a command like:

meshtastic --ch-add test --ch-index 0 --ch-set psk default --ch-set uplink_enabled true

would first add "test" as one of channels 1-7 (assuming one was available), and then set the primary channel to the default PSK and turn uplink on.

With this change, --ch-add won't allow --ch-index to be set along with it, and after adding the channel, will instead set the channel index to that value. That way, the above command would error, but the following:

meshtastic --ch-add test --ch-set psk default --ch-set uplink_enabled true

would, instead, add a channel called "test" with the default PSK and uplink enabled.

I think this is a lot more intuitive and more in line with the usual philosophy of this command line where a single command is intended, generally, to modify only one thing. Hopefully y'all agree!