net-test: packetdrill MPLS support: print MPLS label stacks

Code to print MPLS label stacks in a human-readable format matching
the script syntax:

  mpls (label 123, tc 1, [S], ttl 255) (label 456, tc 1, [S], ttl 255)

Change-Id: I706be8aab089fbad79eabde67125345330728517