dnsmasq: correct sense & usage of dnsseccheckunsigned

dnsmasq: correct sense & usage of dnsseccheckunsigned

dnsmasq v2.80 made 'dnssec-check-unsigned' the default, thus the uci
option was rendered ineffectual: we checked unsigned zones no matter the
setting.

Disabling the checking of unsigned zones is now achieve with the
"--dnssec-check-unsigned=no" dnsmasq option.

Update init script to pass required option in the disabled case.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>