/*
 * Get the file path of a song.
 *
 * This method raises an Xmms::Error exception if XMMS is not running,
 * or an ArgumentError exception if the number of arguments isn't 0 or
 * 1.
 *
 * Examples:
 *   # get the path of the current song
 *   path = remote.file
 *
 *   # get the path of song 23 in the playlist
 *   path = remote.file 23
 *
 */
static VALUE xr_pl_file(int argc, VALUE *argv, VALUE self) {