alsactl: handle detection of new sound card

alsactl: handle detection of new sound card

At present, plug-and-play is not supported in a mode of 'monitor',
thus new sound card is not handled during runtime. This is not happy.

This commit uses Linux-specific inotify(7) to monitor '/dev/snd'
directory. When some files are newly added to the directory,
event dispatcher is suspended. Event sources are scanned again and the
dispatcher continue to run.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>