odhcp6c: add skpriority option

odhcp6c: add skpriority option

Allowing the (kernel) packet priority to be set through UCI.

This makes it straightforward to set some VLAN priority for DHCP
requests through a simple egress qos map. (Avoiding the need for
firewall matching and marking through iptables, which prevents using
flow offloading).

(Such priority tag is a hard requirement for some ISPs, such as Orange
in France).

Depends on: https://github.com/openwrt/odhcp6c/pull/74

Signed-off-by: Pacien TRAN-GIRARD <pacien.trangirard@pacien.net>