Playlists: Fix resuming from control commands with first_index > 0

add_track_to_playlist_unlocked only increased a playlist's
first index as necessary when its position parameter was
negative (i.e. one of the special insert positions was
specified).

A negative value was not stored in the control file, but
was always converted into an absolute position. Thus, any
adjustments to first_index weren't repeated when resuming
from the control file.

In particular, shuffled playlists were affected (in case of
first_index > 0), when inserting at positions <= first_index,
including appending a track to the end of a playlist. This
works by inserting at first_index and increasing first_index
by 1 afterwards.

Similarly, adding tracks in a shuffled fashion could increase
first index, whenever that was the value randomly calculated
for a track position, effectively appending it (I assume this
is on purpose).

To make sure that first_index adjustments are recovered when
resuming from the control file, and to be able to differentiate
between a prepended or appended track, store the special value
PLAYLIST_INSERT_LAST_ROTATED as the insert position in the
control file whenever first_index would have been used before,
and a special position (other than PLAYLIST_PREPEND) was
provided to the function.

Change-Id: I31f26796627fb136daeddd046cb1892bdf1b4014

apps/playlist.h

@@ -62,7 +62,8 @@ enum {
     PLAYLIST_INSERT_FIRST = -4,
     PLAYLIST_INSERT_SHUFFLED = -5,
     PLAYLIST_REPLACE = -6,
-    PLAYLIST_INSERT_LAST_SHUFFLED = -7
+    PLAYLIST_INSERT_LAST_SHUFFLED = -7,
+    PLAYLIST_INSERT_LAST_ROTATED = -8
 };
 
 struct playlist_info