mkswap hasn't had v0 support since commit
d893b120bae5a9bdef4cfda2df5e62867d6b1cd0; it doesn't seem to make sense
to offer it for configuration.
Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>