Change numid properly with external ctl plugins

Change numid properly with external ctl plugins

So far, external ctl plugins don't change numid.  Some apps expect the
non-zero numids with list, and the plugin doesn't work for them.

This patch adds a fake numid to each control based on the offset
number.  The lookup with non-zero numid is supported but is pretty
inefficient.  Eventually the plugin side may be optimized to look
at the numid, too.

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