use a c++ map instead of arrays to store audio stream data
Side effect: audio streams as returned by findAllPids() are now
sorted by pid, helping default selection for MPEG and VDR streams
* avoid race conditions by using a mutex to protect curr_pos, fixing
artefacts during timeshift playback
* make GetPosition() cheaper by interpolating end_pts and only fetching
the "real" end_pts after the file has grown by 10MB
First try at implementing fast forward. Needs more love.
Reverse does not work yet, setting speed to negative values
right now simply resets to "play" mode.
