[check_icmp] Add an option for SO_PRIORITY

[garnier-quentin]

Hi,

Could you add an option to set '802.1p'. It seems we can set it with following option:

int priority = 3;
setsockopt(sfd, SOL_SOCKET, SO_PRIORITY, &priority, sizeof(priority));

It can be easier than a iptables configuration. What do you think about it ?