SoapySDR: Allow channel selection by using the subdev device argument

[holaguz]

This change implements channel selection by specifying the subdev argument in the SoapySDR string. Some programs allow using only one channel (but won't let you select which one), or require you to setup multiple streams to access the higher numbered channels, even if you only care about one.

This has been tested successfully with SoapySDRServer 0.5.2 and Gqrx 2.17.4.

For Gqrx, using the string soapy=0,driver=lime,serial=XXXXXXXXXXXXX,subdev=0 selects the first channel, and setting subdev=1 uses the second channel.

On SoapySDRServer it was tested using satnogs-client, specifying the arguments as driver=remote,remote:driver=lime,serial=XXXXXXXXXXXXX,subdev=<channel>

You can see https://network.satnogs.org/observations/11590367/ and https://network.satnogs.org/observations/11590376/ for two satellite observations made using channel 0 (VHF) and channel 1 (UHF) (check the metadata tab)

[holaguz]

renamed subdev to channel and specified that it should be a list of integers separated by spaces.

added an error log when the channel string is invalid:
[ERROR] Couldn't parse 'channels' string 'a 1': it should be a list of integers separated by spaces.