Prepare on-demand decoder module loading by adding a module_name member
to struct proto_names and introducing the related load function.

After this patch of the series the decoder modules are still loaded
unconditionally.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>