mpegtsmux: Don't create streams with reserved PID
There are quite a few reserved PID in the various MPEG-TS (and derivate) specifications which we should definitely not use. Those PID have a certain meaning and purpose.
Furthermore, a lot of the code in the muxer implementation also makes assumption on the purpose of streams based on their PID.
Therefore, when requesting a pad with a specific PID, make sure it is not a restricted PID.