[c-nsp] NPE-G1 support for jumbo frames

[Christopher Hunt]

we run MPLS on the G-1 gig ports over copper.  we use the MPLS MTU 
command to override the interface MTU.  It allows the original L3 packet 
to be 1500 PLUS adds MPLS headers, technically exceeding the interface 
MTU.  An MPLS MTU of 1512 allow for up to 3 4-byte MPLS labels (1 for 
PE, 2 for P, 3 for MPLS-TE)

|interface GigabitEthernet0/1||
|| ip address x.x.x.x y.y.y.y
|| ...||
|| mpls ip
|| mpls mtu 1512|||