Add -z "TOS" option to ping6, to test DSCP/ECN values

[rscheff]

Summary

ping has the option to add the (deprecated) TOS byte
using the -z option. Adding the same option, with the
same (deprecated) Traffic Class Byte (nowadays actually
DSCP and ECN fields) to ping6 to validate proper QoS
processing in network switches.

Test Plan

Run "tcpdump -vvv -nn -i lo0 -xx" and verify, that
the ICMP6 request has the provided data for the traffic
class in the 5th and 6th octet, right after the IP
version "high nibble" (6) in the 5th octet.