alsactl: add the daemon mode

alsactl: add the daemon mode

For the plug-and-play hardware, like USB devices, it may be helpful
to manage the sound state periodically, before the devices are removed.

This change implements new commands 'daemon' and 'rdaemon' to save
the sound state in defined intervals when the sound controls are changed.

The udev rules can notify the daemon using the 'kill' or 'nrestore'
commands to rescan available cards in the system.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>