bug#16134: Add support for atari partition tables, v3

Hi!

This is my third revision of this patch series which enables both the
tests for partition flags (tests/t3310-flags.sh) as well as the
test _implemented_disk_label from the libparted test suite.

For the latter to work, I needed to move get_sector_size() from disk.c
to common.c so it could be used to test the sector size and make sure
the tests are only performed with 512 byte sectors on atari partition
tables.

Adrian