zebra: Cleanup mpls handling to allow a NEXTHOP_TYPE_IFINDEX

zebra: Cleanup mpls handling to allow a NEXTHOP_TYPE_IFINDEX

Add the ability for the nexthops to be a NEXTHOP_TYPE_IFINDEX.
Since we are using this code for L3vpn pop and forward operations
and we know that the lo or vrf device name must exist we
trust that it is correct.

Update display to show the correct data with a 'show mpls table'

Update the mpls install into the kernel to treat
NEXTHOP_TYPE_IFINDEX as special and we do not need
to pass in the nexthop label.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>