Explicit-null packet classification using the EXP value from MPLS explicit-null labels

The default classification for explicit-null packets is based on the payload (IPv4 or IPv6 DSCP bits). Devices support a new CLI option, [explicit-null-cos inet|inet6] at the [edit forwarding-options] hierarchy level, that makes the packet classification based on the MPLS EXP value rather than on the payload, thus preserving the MPLS classification of the packet.