V4L/DVB (9004): S2API: Implement GET/SET handing to the demods

The frontends will be notified (if they chose) of all _get and _set commands
so they can help determine result or action. Results are now returned
to userspace correctly.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>