net-test: packetdrill MPLS support: parse MPLS label stacks in scripts

Code to parse script packets using an encapsulation layer that is an
MPLS label stack.

The syntax looks like:

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

This is borrowed from the syntax from tcpdump, which is:

  MPLS (label 123, exp 1, [S], ttl 255) (label 456, exp 1, [S], ttl 255)

We use "tc" because RFC 5462 renamed the EXP ("experimental") field to
the TC ("traffic class") field. We use lower-case "mpls" instead of
"MPLS" for consistency with other packetdrill protocols, and for ease
of typing.

Change-Id: I2e797aead8605da9fe3f0461594cf7273d4ef0ec