jack: Support to connect multiple JACK ports with same ALSA channel

jack: Support to connect multiple JACK ports with same ALSA channel

The following example will connect ALSA channel 0 to JACK port
"system:playback_1" and "system:playback_3" and ALSA channel 1 to JACK
port "system:playback_2" and "system:playback_4":

pcm.jack {
	type jack
	playback_ports {
		0 [ system:playback_1 system:playback_3 ]
		1 [ system:playback_2 system:playback_4 ]
	}
}

The old syntax with only one port for one channel is still supported:
playback_ports {
	0 system:playback_1
	1 system:playback_2
}

Without this patch an additional JACK client has to be used to
automatically connect the second JACK port but this could take some
time. Therefore it misses for example the first audio period on the
second port.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>