luci-mod-admin-full: correct logic for dnsmasq boguspriv keyword

Prevents deletion of the UCI dns boguspriv keyword and explicitly sets it to
either 0 or removes it as the LEDE/Openwrt dnsmasq.conf default is 1 - not 0.

This allows boguspriv to be turned off from the GUI.

Signed-off-by: Warren Linton <warren@linton.id.au>
[Jo-Philipp Wich: squashed commits, updated commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>