rfc: Change behavior of skip to previous song

@ejurgensen this changes the skip to previous song command to start the current song from the beginning, if it is playing for more than 3 seconds. Thats how iTunes handles the prev-command (and i would expect it to work).

Unfortunately i found a problem using local audio. The progress indicator always started at around 0:24 instead of 0:00 (the song was playing correctly from the beginning). After applying the first commit it was impossible to skip to the previous song, because "pos_sec" was always greater 3. The second commit solved this in my case, but i am not sure if this is the correct solution (as the comment above my code change states it was on purpose) and it doesn't break anything.

Could you have a look at it? Were there reports from other users that the progress indicator was wrong when using local audio?
I am not sure if it is worth the trouble, if we can't get a clear answer if the second commit is correct ...
Would it be possible to use get_status() for this? I would prefer that, as it avoids duplicate, but slightly different, calculations of playing time.