Add options to configure network ttl

[mxvin]

some network env sometimes enforcing / expect device to have certain TTL. When network is re-routed/nat, the traffic from tethered device is ttl +1 and make tethered device not connected to upstream network.

something like iptables -w -t mangle -I PREROUTING -i <out net iface> -j TTL --ttl-set 129 or <...> --ttl-inc 1 would do.

should be added and configurable if needed context, If you not familiar of this kind of problem https://forum.openwrt.org/t/iptables-ttl-mangling-doesnt-work-with-bridged-setup/32708

[Yorizuka]

This would be really useful, I really hope someone adds this!