Fix and improve the slots option handling. The sound core tries to
find the slot with the given module name first and assign if it's
still available. If all pre-given slots are unavailable, then try
to find another free slot.
Also, when a module name begins with '!', it means the negative match:
the slot will be given for any modules but that one.
Signed-off-by: Takashi Iwai <tiwai@suse.de>