MPLS support on services PICs

Adds MPLS label pop support for services PICs on Junos OS routers. Previously, all MPLS traffic would be dropped at the services PIC. No changes are required to CLI configurations for this enhancement. In-service software upgrade (unified ISSU) is supported for tag next hops for MPLS on services PIC traffic, but no support is provided for tags over IPv6 packets or labels on multiple gateways.