TCP checksum not set

[fujiapple852]

When sending TCP packets the checksum is not being set which results in the packets being dropped by routers.

Trippy currently uses pnet layer4 and to set the checksum we need to know both the destination and source IP address. We need to obtain the source IP address using the technique described in datalink::interfaces, but as the documentation notes:

is not guaranteed to work on each system

Later, Trippy will move to use layer3 or layer2 channels (#46) and the source interface can be chosen explicitly (#42).

Note that for UDP we deliberately do not set the checksum as, in some configurations, the checksum is used as to track the sequence number (MTR behaviour).