axfer: add a sub-command to print list of PCMs/devices

axfer: add a sub-command to print list of PCMs/devices

Original aplay implementation has a feature to output two types of list;
devices and PCMs. The list of devices is a result to query sound card and
pcm component structured maintained in kernel land. The list of PCMs is a
result to parse runtime configuration files in alsa-lib. Entries in the
former list is corresponding to ALSA PCM character device
('/dev/snd/pcm%uC%uD[p|c]'), while entries in the latter list includes
some 'virtual' instances in application runtime.

This commit adds an implementation for the above functionality. This is
executed by taking 'list' sub-command. A 'device' option has the same
effect as '--list-devices' and '-L' of aplay. A 'pcm' option has the same
effect as '--list-pcms' and '-l' of aplay. In both cases, an additional
option is required for stream direction. Below is examples of new command
system for this sub-command.

$ axfer list device -C (= arecord --list-devices)
$ axfer list pcm -P    (= aplay -l)

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>