tayga: Add Netifd support

tayga: Add Netifd support

This patch integrates tayga with netifd. Parametres are nearly same as
with the older scripts. Support for static mapping of IPv4<=>IPv6 addresses
is missing. Example configuration:

config interface 'nat64'
        option proto 'tayga'
        option prefix 64:ff9b::/96
        option dynamic_pool 10.128.0.0/24
        option ipv4_addr 10.128.0.1	#address of the TAYGA itself
        option ipv6_addr 2001:470:5990::64
        option ipaddr  192.168.1.1 #optional address of TUN interface
        option ip6addr 2001:db8::1

Signed-off-by: Ondrej Caletka <ondrej@caletka.cz>