tayga: add options for static mapping

tayga: add options for static mapping

The commit adds options for static mapping, which is essential for
464xlat(clat) usage. But be noted that routes and firewall rules have
to be configured manually, for now.

To use tayga as clat/nat46:
0. specify $ipv4_addr, $prefix, $map_{ipv4,ipv6} properly, and
   set $noroutes to 1 to prevent creating routes for nat64
1. add custom IPv4 routes to tayga, possibly with a reasonable metric
   for default route
2. add a SNAT firewall rule for processing IPv4 traffic destined for
   tayga
3. add firewall rules to allow FORWARD traffic between tayga and WAN6
4. add a $map_ipv6 route to tayga for guiding return traffic

Signed-off-by: Hung-I Wang <redacted>