* Fix a memory leak / crasher

* Start putting together all interface strings (Refs:#703)

* Fix a nasty playlist crash

 * Add atrack, vtrack and strack commands. Those can be use to display/change audio, video and subtitle tracks.
 * Fix compile warning in enqueue command.

  This should help identify the producing encoder and fix some issues with codec specific problems

put mono audio in both (left/right) channels. For some reason AOUT_CHAN_CENTER doesn't work. Somebody explain this to me, please.

* Does nothing useful at the moment (only useless device probing)
* The purpose would be to make an access-demux and an access sub-module reusing part of the code
* Access to a webcam or an analog card without an encoder (bt878) would help the development

* browsed through all code files starting with A to F and added non-blocking intf_UserFatal calls next to msg_err where I think that it is necessary.
I tried to select the most important msg_errs. If I missed your favourite, feel free to add it.
Feedback on the wording and the placement of the interaction calls is strongly welcomed.

For the records: I'm using some general topics for most of the dialogues to produce less work for the translators. The current ones go like this:
"Audio output failed", "Network interaction failed", "Streaming / Transcoding failed", "Audio filtering failed", "File reading failed", "Playback failure"

More to come.