alsactl: Read only *.conf files when a directory is passed via INCLUDE

alsactl: Read only *.conf files when a directory is passed via INCLUDE

When alsactl init is invoked and a directory path is passed to INCLUDE
command in the config file, read only *.conf files in that directory.
This will avoid reading backup files or invalid files that have been
created accidentally.

Also by using scandir() with alphasort(), alsactl reads the files in
alphabetical order.  Thus it's highly recommended to use some number
prefix to the file name for assuring the order.

Signed-off-by: Takashi Iwai <tiwai@suse.de>