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