axfer: add support for MMAP PCM operation

axfer: add support for MMAP PCM operation

In alsa-lib PCM API, data frames can be handled in mapped page frame,
instead of calling any system calls.

This commit support for this type of operation. To reduce CPU usage,
this commit uses 'snd_pcm_wait()' to wait for event notification.

Below lines are examples to execute:
$ axfer transfer -M -P -d 2 -D hw:0,3 /dev/urandom -f dat -vvv
$ axfer transfer -M -C -d 2 -D hw:1,0 /dev/null -r 48000 -vvv

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