media: dvb: Out-of-Band (OOB) commands support

Extend demux API with ability to push OOB commands to the DVR device
during playback from memory. OOB command is then received by all running
filters. Two types of OOB commands are supported:
1. Data markers - these allow user to mark with an identifier certain
points in the source stream, and be notified via the demux filters when
those are reached.
2. End-of-Stream (EOS) - allow user to declare source stream has reached
its end. This will make pending data readable to user and block any more
data from being output.

Change-Id: I3b3574718fa975ff7626663c38ed3b8f9e1853e4
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>