conf: Allow for a directory to be given as a config file.

conf: Allow for a directory to be given as a config file.

When this is done, *.conf files can be placed in that directory and they
will be processed by as if they were included directly.

A directory (typically /usr/share/alsa/alsa.conf.d/) has been
added into the distribution.

v2: Used existing conf syntax rather than processing via autotools
v3: Split file loading into separate function and made error handling
more consistent.

Signed-off-by: Colin Guthrie <colin@mageia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>