libfdisk: (gpt) allow to specify attr bit by API

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: move get_parttypes to label API

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: final parttype API cleanup

Signed-off-by: Karel Zak <kzak@redhat.com>