Unlike all the other jack backends which allow setting the number of inputs and
outputs with -i & -o, the dummy backend uses -P & -C for this. Make the jackd
command line use these options when the dummy backend is requested with a
specified input or output channel count.
The dummy jackd backend doesn't require a device to be specified, so much
of the error checking in get_jack_command_line_string() is irrelevant, if
not actively wrong, when the dummy backend is specified.
Only perform the checks if the chosen jack backend is not the dummy.
