luci-mod-network: Add flag for address_as_local

luci-mod-network: Add flag for address_as_local

'Addresses' option entries in the general tab of DHCP and DNS settings are
sometimes (I'm not sure about the exact conditions) passed to upstream resolver,
bypassing local resolution.

A workaround for a small list of domains is to add them to 'Resolve these
locally' option, but this is very tedious to do, and dnsmasq limits it to 1024
characters.

openwrt/openwrt#18610 adds behaviour to add addresses to the local resolution
list automatically, and this commit adds a flag to LuCI to control that.

Signed-off-by: Marko Zajc <[email protected]>