odhcp6c: add -K option to set packet kernel priority

odhcp6c: add -K option to set packet kernel priority

This adds a command line option (-K) to set the packet kernel
priority.

This makes it straightforward to set some VLAN priority for DHCP
requests through an 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).

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