V4L/DVB (9072): S2API: Add DTV_API_VERSION command

This allows application developers to query the dvb-core API version
dynamically, helping developers understand whether certain features
will be available.

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