Protocol: extend Data-Level Length field

[matttbe]

When GSO is used, we can now have packets with a size > 2¹⁶ (see commit 7c4e983) which is not compatible with MPTCP's DLL (Data-Level Length) field (see issue #450 and 9fce92f) limited to 2¹⁶.

Paolo suggested to increase MPTCP's DLL field by reusing (or moving?) the Checksum field.

We could have an extension of the MPTCPv1 protocol using one unused flag from the MPC to negotiate this option. If both ends support it, we can add up to 2 bytes to this field.

[matttbe]

Note from a previous weekly meeting:

• We depend on TCP for that where the limit is currently around .5MB
• We don't need more than 24bits but what are we going to do with the remaining 8 bits?
• Would be part of a potential MPTCPv2