Don't fail jackd command line creation for jack dummy backend.

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.
Fix setting playback & capture channel counts for jackd dummy backend.

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.
Don't ever pass -d for device name to dummy jackd driver
Report an error to stderr if creation of jack command line fails.

Output a simple message to stderr if get_jack_command_line_string() still
fails for any reason.