allow setting local and remote port for UDP probing

allow setting local and remote port for UDP probing

This allows for testing over a single network path in a load balanced
network where flows are hashed on the ips and ports (relatively common), the
default behavior is to use the destination port as the sequence number when
doing UDP probes, so you see (more or less) an average of all the paths.

If the remote port is set, we need to use the checksum field to store
sequence and adjust the paylod so the checksum is valid; therefore we need
to have the source address, as it's part of the checksum calculation.  We
also need to increase the minimum packet size so we have at least 2 bytes of
payload.