S-Video/Composite Inputs require an external channel change script

[stuartm]

Now I'm not sure whether the solution to this problem is just documentation, or whether we can't come up with a much better way of handling this. I spent the best part of a day to figure out that the 'Optional' field in Input Connections wasn't optional after all for the composite input of my PVR 150.

In the end, using /bin/true solved the immediate problem. This isn't documented that I could find. Even then it doesn't make much sense to require the user specify /bin/true if they don't have an IR blaster (etc) if we could just handle this internally somehow.

[danielk]

(In [14762]) Fixes #3455. Don't try to change channels on inputs labeled 'Composite' or 'S-Video', abd always set the tunechan when it is configured.

Instead of failing with opaque error conditions when we can't tune a 'Composite' or 'S-Video' input this makes channel.cpp print out an informative message about setting the external channel changing script and does not try to tune hardware which contains no tuner.