[player] Better fix for resuming playback af long pause (issue #766)

The fix in commit 3928ab6 broke resuming from an underrun, since it meant that
pb_resume() would flush the input buffer. With this fix it is possible to call
input_resume(), which will not flush the buffer if the source is already open.

Also renamed some functions in player.c for consistency.

src/input.h

@@ -196,6 +196,17 @@ input_buffer_full_cb(input_cb cb);
 int
 input_seek(uint32_t item_id, int seek_ms);
 
+/*
+ * Same as input_seek(), except if the item is already being read we don't do
+ * anything (no flush & seek)
+ *
+ * @in  item_id  Queue item id to start playing
+ * @in  seek_ms  Position to start playing
+ * @return       Actual seek position if seekable, 0 otherwise, -1 on error
+ */
+int
+input_resume(uint32_t item_id, int seek_ms);
+
 /*
  * Same as input_seek(), just non-blocking and does not offer seek.
  *