Correct ltfs_check_medium handling of partitions ending in data+fm

Alter ltfs_check_medium to do a more thorough detection of the state of
the partitions after the detected indices and set the eod/append
position appropriately if the partition ends in data + filemark

Fixes #272

src/libltfs/ltfs_internal.c

@@ -992,7 +992,7 @@ int _ltfs_make_lost_found(tape_block_t ip_eod, tape_block_t dp_eod,
  * @return 0 if eod is the appropriate append address, >0 for the absolute block address to append
  *         at, <0 on error
  */
-int _ltfs_find_append_blk_after_idx(struct device_data *dev, tape_partition_t part, tape_block_t block) {
+static int _ltfs_find_append_blk_after_idx(struct device_data *dev, tape_partition_t part, tape_block_t block) {
 	unsigned int n_fm_after = 0;
 	struct tc_position idx_pos;
 	struct tc_position final_fm_pos;