devel/libisofs: Update to 1.5.6

Major changes:
    * Bug fix: iso_write_opts_set_part_like_isohybrid() did not cause a MBR
      partition table if the partitions are data files in the ISO
      rather than appended
    * Bug fix: The lseek methods of IsoFileSource for local filesystem and loaded
      ISO returned libisofs error codes as positive off_t numbers
    * Bug fix: Freshly cloned data files from imported image were not marked as
      imported. Thanks to Ivan Shmakov. (Closes: #1022851)
    * Bug fix: Size of further CE area was calculated wrong if its CE entry ended
      exactly at a block boundary
    * New iso_write_opts_set_system_area() option bits 16:
      GPT "Legacy BIOS bootable" and 17: GPT writable
    * New API calls iso_assess_written_features(), iso_read_image_feature_named(),
      iso_read_image_features_text()
    * Allowed lseekable device files with iso_tree_add_new_cut_out_node().
      Proof-of-concept by Ivan Shmakov.
    * New API call iso_write_opts_set_max_ce_entries()

Full changelog: https://dev.lovelyhq.com/libburnia/libisofs/src/branch/master/ChangeLog

Port changes:
    * Update WWW url
    * Reorder Makefile to make linters happy