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

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.

Reviewed by: tuexen
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26384