add EN50494 ("unicable") support to tvheadend

see https://tvheadend.org/boards/5/topics/8586 and
https://tvheadend.org/issues/1752

Note that I am not an active tvheadend user, I just use it occasionally for testing third party software.
So even though it is unlikely that I'm falling off the edge of the planet soon, I'll probably not notice problems in this code by myself.
I will of course still try to fix issues that arise.