net-test: packetdrill encap support: main encapsulation engine

The main engine for handling encapsualted packets in packetdrill,
including:

- parsing scripts with encapsulation
- parsing packets with encapsulation
- injecting packets with encapsulation

Encapsulated packets look like the following, which has an IP header,
GRE header, and then regular IP/TCP header:

+0 > ipv4 1.1.1.1 > 2.2.2.2: gre: . 1:1001(1000) ack 1

See other patches in the series for more detailed examples.


Change-Id: Ia8c7580e268f5b0347bd9d3fdc73b096f751a79d