axfer: add options for plugins in alsa-lib

axfer: add options for plugins in alsa-lib

As of 2017, two userspace library implementations are known; alsa-lib and
tinyalsa. The latter is simple I/O library to use ALSA PCM interface. On
the other hand, alsa-lib is more complicated than it. This is because it's
designed to add features to transmission of data frames; e.g. sample
resampling. To achieve this, alsa-lib has its configuration space and
plugin system.

In aplay, some options are implemented as a flag for the plugins in
alsa-lib. The flag is given to snd_pcm_open(). This commit adds support
for the flags.

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