libfdisk: make script header parser more pedantic

Signed-off-by: Karel Zak <kzak@redhat.com>
sfdisk: leave on error

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: fix script next_token()

Signed-off-by: Karel Zak <kzak@redhat.com>
tests: add sfdisk GPT tests

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