ucm: Set default include path

Many UCM profiles include the UCM profile components under ucm/*
subdirectories and thusly put <searchdir:ucm> at each place.  This is
rather cumbersome.

This patch makes the UCM parser to set the default include path, so
that each profile no longer needs to set searchdir.  All the
<searchdir:ucm> lines currently found in the profiles are removed
gracefully, too.

For the needed implementation, a new helper,
_snd_config_load_with_include() is introduced.  It's not exported,
only for the use inside alsa-lib.

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